I am still getting this bug with the proposed version. Ubuntu 20.04 on a Lenovo machine.

I hope I updated correctly, I followed the instructions linked above to activate the "proposed" repository and installed the package with "sudo apt install mutter/focal-proposed", then deactivated the "proposed" repository and rebooted my machine. For the record, "dpkg --status mutter" shows "Version: 3.36.4-0ubuntu0.20.04.2", so I think I did it correctly. I did not touch gnome-shell. "gnome-shell --version" shows 3.36.3.

I used the touchscreen and triggered the bug after just a few minutes. gnome-shell crashed and some familiar messages were dumped in the syslog:

Aug 11 10:51:24 c930 gnome-shell[5197]: Object Meta.BackgroundActor (0x55be2f3b63f0), has been already deallocated — impossible to access it. This might be caused by the object having been destroyed from C code using something such as destroy(), dispose(), or remove() vfuncs.
Aug 11 10:51:24 c930 gnome-shell[5197]: == Stack trace for context 0x55be2bb6a6f0 ==
Aug 11 10:51:24 c930 gnome-shell[5197]: #0   7fff7da0d270 b   resource:///org/gnome/gjs/modules/core/overrides/GObject.js:571 (2255897b6c40 @ 25)
Aug 11 10:51:24 c930 gnome-shell[5197]: #1   55be2caa60a0 i   /usr/share/gnome-shell/extensions/desktop-icons@csoriano/desktopGrid.js:209 (2ec7edda91f0 @ 85)
Aug 11 10:51:24 c930 gnome-shell[5197]: #2   55be2caa6018 i   /usr/share/gnome-shell/extensions/desktop-icons@csoriano/desktopGrid.js:148 (2ec7edda4f88 @ 12)
Aug 11 10:51:24 c930 gnome-shell[5197]: JS ERROR: Error: Argument 'instance' (type interface) may not be null#012_init/GObject.Object.prototype.disconnect@resource:///org/gnome/gjs/modules/core/overrides/GObject.js:571:24#012_onDestroy@/usr/share/gnome-shell/extensions/desktop-icons@csoriano/desktopGrid.js:209:45#012_init/<@/usr/share/gnome-shell/extensions/desktop-icons@csoriano/desktopGrid.js:148:44
Aug 11 10:51:24 c930 gnome-shell[5197]: clutter_actor_iter_next: assertion 'ri->age == ri->root->priv->age' failed
Aug 11 10:51:27 c930 gnome-shell[5197]: **
Aug 11 10:51:27 c930 gnome-shell[5197]: mutter:ERROR:../src/backends/x11/meta-stage-x11.c:870:meta_x11_get_stage_window: assertion failed: (META_IS_STAGE_X11 (impl))
Aug 11 10:51:27 c930 gnome-shell[5197]: Bail out! mutter:ERROR:../src/backends/x11/meta-stage-x11.c:870:meta_x11_get_stage_window: assertion failed: (META_IS_STAGE_X11 (impl))
Aug 11 10:51:27 c930 gnome-shell[5197]: GNOME Shell crashed with signal 6
Aug 11 10:51:27 c930 gnome-shell[5197]: == Stack trace for context 0x55be2bb6a6f0 ==
Aug 11 10:51:41 c930 whoopsie-upload-all[7230]: Collecting info for /var/crash/_usr_bin_gnome-shell.1000.crash...
Aug 11 10:51:41 c930 whoopsie-upload-all[7230]: Marking /var/crash/_usr_bin_gnome-shell.1000.crash for whoopsie upload
Aug 11 10:51:41 c930 whoopsie[2089]: [10:51:41] Parsing /var/crash/_usr_bin_gnome-shell.1000.crash.
Aug 11 10:51:42 c930 whoopsie[2089]: [10:51:41] Uploading /var/crash/_usr_bin_gnome-shell.1000.crash.
Aug 11 10:51:42 c930 systemd[2217]: gnome-shell-x11.service: Main process exited, code=dumped, status=6/ABRT
Aug 11 10:51:42 c930 systemd[2217]: gnome-shell-x11.service: Failed with result 'core-dump'.
Aug 11 10:51:42 c930 systemd[2217]: gnome-shell-x11.service: Scheduled restart job, restart counter is at 2.
Aug 11 10:51:42 c930 whoopsie-upload-all[9369]: /var/crash/_usr_bin_gnome-shell.1000.crash already marked for upload, skipping

I'm uploading the output of "apport-retrace -R -s _usr_bin_gnome-shell.1000.crash", if the information is incomplete please let me know what I can do get better data.