I am also seeing this same problem on a Raspberry Pi4 4GB.

I purchased the Pi last November and installed from the then-available 2019-09-26-raspbian-buster.zip but routinely keep it updated (apt update / apt upgrade).

The Pi was rock solid and did not exhibit any problems until 2020-02-24 when I applied some updates followed by a "sudo reboot" but the boot process did not complete. The Pi runs headless so I don't know whether it never actually completed the shutdown, or failed to reboot. I power-cycled the device and it came back OK.

Another set of updates were applied on 2020-03-11 and the subsequent reboot completed without incident so I assumed the earlier problem was a one-off.

Then, on 2020-03-14 the Pi froze. Wouldn't respond to pings, ssh or vnc. It is Ethernet-connected but the WiFi interface is active and it did not occur to me to try connecting via WiFi. I just did another power-cycle and then started nosing around through the logs. Which brought me to:

Mar 14 12:48:16 iot-hub avahi-daemon[361]: Withdrawing address record for 192.168.132.60 on eth0.
Mar 14 12:48:16 iot-hub avahi-daemon[361]: Leaving mDNS multicast group on interface eth0.IPv4 with address 192.168.132.60.
Mar 14 12:48:16 iot-hub avahi-daemon[361]: Interface eth0.IPv4 no longer relevant for mDNS.

and, from there, to this post.

The main reasons I'm adding a comment to this post are (a) as a "me too" as of March 2020, and (b) in case the pattern of the RPi being perfectly OK until 2020-02-24 causes someone to say "ahah!"

The updates in question were (from the history log): Upgrade: libpython3.7-minimal:armhf (3.7.3-2, 3.7.3-2+deb10u1), libraspberrypi-bin:armhf (1.20200114-1, 1.20200212-1), libopenjp2-7:armhf (2.3.0-2, 2.3.0-2+deb10u1), rc-gui:armhf (1.34, 1.36), libcom-err2:armhf (1.44.5-1+deb10u2, 1.44.5-1+deb10u3), libcups2:armhf (2.2.10-6+deb10u1, 2.2.10-6+deb10u2), libraspberrypi-dev:armhf (1.20200114-1, 1.20200212-1), libraspberrypi-doc:armhf (1.20200114-1, 1.20200212-1), python3.7-venv:armhf (3.7.3-2, 3.7.3-2+deb10u1), raspberrypi-ui-mods:armhf (1.20200127, 1.20200218), qt5-gtk-platformtheme:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libfm4:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13), libsystemd0:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), raspi-config:armhf (20200205, 20200207), libfm-modules:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13), libfm-extra4:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13), mariadb-common:armhf (1:10.3.17-0+deb10u1, 1:10.3.22-0+deb10u1), binutils:armhf (2.31.1-16+rpi1, 2.31.1-16+rpi2), e2fsprogs:armhf (1.44.5-1+deb10u2, 1.44.5-1+deb10u3), sudo:armhf (1.8.27-1+deb10u1, 1.8.27-1+deb10u2), libqt5dbus5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libqt5sql5-sqlite:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), python3-pil:armhf (5.4.1-2, 5.4.1-2+deb10u1), libpython3.7:armhf (3.7.3-2, 3.7.3-2+deb10u1), python3.7:armhf (3.7.3-2, 3.7.3-2+deb10u1), openssh-sftp-server:armhf (1:7.9p1-10+deb10u1, 1:7.9p1-10+deb10u2), libqt5widgets5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libfm-gtk4:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13), udev:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), python3.7-dev:armhf (3.7.3-2, 3.7.3-2+deb10u1), binutils-arm-linux-gnueabihf:armhf (2.31.1-16+rpi1, 2.31.1-16+rpi2), libpython3.7-stdlib:armhf (3.7.3-2, 3.7.3-2+deb10u1), libudev1:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), python3.7-minimal:armhf (3.7.3-2, 3.7.3-2+deb10u1), libqt5xml5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libqt5printsupport5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), raspberrypi-kernel:armhf (1.20200114-1, 1.20200212-1), libss2:armhf (1.44.5-1+deb10u2, 1.44.5-1+deb10u3), libqt5concurrent5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libext2fs2:armhf (1.44.5-1+deb10u2, 1.44.5-1+deb10u3), raspberrypi-bootloader:armhf (1.20200114-1, 1.20200212-1), libexif12:armhf (0.6.21-5.1, 0.6.21-5.1+deb10u1), libqt5gui5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libimobiledevice6:armhf (1.2.1~git20181030.92c5462-1, 1.2.1~git20181030.92c5462-2), gldriver-test:armhf (0.4, 0.5), ssh:armhf (1:7.9p1-10+deb10u1, 1:7.9p1-10+deb10u2), systemd-sysv:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), guile-2.2-libs:armhf (2.2.4+1-2, 2.2.4+1-2+deb10u1), realvnc-vnc-server:armhf (6.6.0.41949, 6.7.1.42348), libpython3.7-dev:armhf (3.7.3-2, 3.7.3-2+deb10u1), libpam-systemd:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), libmatroska6v5:armhf (1.4.9-1, 1.4.9-1+deb10u1), systemd:armhf (241-7~deb10u2+rpi1, 241-7~deb10u3+rpi1), libwebkit2gtk-4.0-37:armhf (2.26.3-1~deb10u1+rpi1, 2.26.4-1~deb10u1+rpi1), libqt5core5a:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), piwiz:armhf (0.8, 0.9), libraspberrypi0:armhf (1.20200114-1, 1.20200212-1), openssh-server:armhf (1:7.9p1-10+deb10u1, 1:7.9p1-10+deb10u2), openssh-client:armhf (1:7.9p1-10+deb10u1, 1:7.9p1-10+deb10u2), libmysofa0:armhf (0.6~dfsg0-3, 0.6~dfsg0-3+deb10u1), rpi-chromium-mods:armhf (20200124, 20200207), binutils-common:armhf (2.31.1-16+rpi1, 2.31.1-16+rpi2), rp-prefapps:armhf (0.15, 0.17), libqt5opengl5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), libmariadb3:armhf (1:10.3.17-0+deb10u1, 1:10.3.22-0+deb10u1), libfm-data:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13), libbinutils:armhf (2.31.1-16+rpi1, 2.31.1-16+rpi2), libgnutls30:armhf (3.6.7-4, 3.6.7-4+deb10u2), libqt5network5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), pcmanfm:armhf (1.3.1-1+rpt17, 1.3.1-1+rpt18), libcupsimage2:armhf (2.2.10-6+deb10u1, 2.2.10-6+deb10u2), libgnutls-dane0:armhf (3.6.7-4, 3.6.7-4+deb10u2), libjavascriptcoregtk-4.0-18:armhf (2.26.3-1~deb10u1+rpi1, 2.26.4-1~deb10u1+rpi1), python-pil:armhf (5.4.1-2, 5.4.1-2+deb10u1), libqt5sql5:armhf (5.11.3+dfsg1-1+rpi1, 5.11.3+dfsg1-1+rpi1+deb10u3), docker-ce:armhf (5:19.03.5~3-0~raspbian-buster, 5:19.03.6~3-0~raspbian-buster), base-files:armhf (10.3+rpi1+deb10u2, 10.3+rpi1+deb10u3), docker-ce-cli:armhf (5:19.03.5~3-0~raspbian-buster, 5:19.03.6~3-0~raspbian-buster), libfm-gtk-data:armhf (1.3.1-1+rpt12, 1.3.1-1+rpt13)

The RPi has had the "turn off IPv6" lines mentioned in earlier comments in sysctl.conf since the machine was first installed. The reason had nothing to do with this kind of problem - I wanted to stop IPv6 DNS requests cluttering-up my PiHole logs and killing IPv6 entirely seemed the best way to do that.

My DHCP server is a DrayTek router. It had been set to a 3-day lease time. I've just upped that to 10 days for no particular reason other than earlier postings suggest longer times are better. It's only a home network and I'm hardly going to run out of leases.