If the list of files accessible via the shared-memory interface slot changes, the apparmor profile for the plugs aren’t updated after the slot snap is refreshed. If you disconnect and reconnect the slot, then the profile is updated appropriately. Other interfaces such as system-files do not behave this way and automatically update the profile.
In the example below, test-snap_version1 has a slot allowing read access to /dev/shm/foo. version2 of the snap adds access to /dev/shm/bar. And test-snap-plug has a plug to the shared memory slot provided by test-snap.
$ sudo sh -c 'echo "Foo!" > /dev/shm/foo'
$ sudo sh -c 'echo "Bar!" > /dev/shm/bar'
$ snap install test-snap_version1_all.snap --dangerous
test-snap version1 installed
$ snap install test-snap-plug_version1_all.snap --dangerous
test-snap-plug version1 installed
$ sudo test-snap-plug.sh -c 'cat /dev/shm/foo' # Expected Failure
cat: /dev/shm/foo: Permission denied
$ snap connect test-snap-plug:foobar-plug test-snap:foobar-slot
$ sudo test-snap-plug.sh -c 'cat /dev/shm/foo'
Foo!
$ sudo test-snap-plug.sh -c 'cat /dev/shm/bar' # Expected Failure
cat: /dev/shm/bar: Permission denied
$ snap install test-snap_version2_all.snap --dangerous
test-snap version2 installed
$ sudo test-snap-plug.sh -c 'cat /dev/shm/bar' # This should work
cat: /dev/shm/bar: Permission denied
$ snap disconnect test-snap-plug:foobar-plug
$ snap connect test-snap-plug:foobar-plug test-snap:foobar-slot
$ sudo test-snap-plug.sh -c 'cat /dev/shm/bar'
Bar!
Expected Behavior: After installing the snap version with the updated slot, the snap with the plug should be able to access /dev/shm/bar without having to disconnect and re-connect the slot/plug.