Indeed I can see the rules you mention in /etc/apparmor.d/abstractions/base, which is included by /var/lib/snapd/apparmor/profiles/snap.chromium.chromium.
However I can reliably reproduce the issue, and I'm seeing the following denial:
AVC apparmor="DENIED" operation="open" profile="snap.chromium.chromium" name="/home/ubuntu/.Private/" pid=11167 comm="pool" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
Indeed I can see the rules you mention in /etc/apparmor. d/abstractions/ base, which is included by /var/lib/ snapd/apparmor/ profiles/ snap.chromium. chromium.
However I can reliably reproduce the issue, and I'm seeing the following denial:
AVC apparmor="DENIED" operation="open" profile= "snap.chromium. chromium" name="/ home/ubuntu/ .Private/ " pid=11167 comm="pool" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000