With an update to wl_output interface version 4 WAM doesn't really come up, and it seems like the wayland client handling in chromium might have some issues handling it.
Some discussions over IRC and minor investigation revealed that the wayland client is actually using the version provided by the compositor, rather using setting a fixed maximized version which should correctly handle.
https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27949 < this is the change for wl_output interface version 4.
Further more with the agl-shell protocol update, from
https://gerrit.automotivelinux.org/gerrit/c/src/agl-compositor/+/27859, I am seeing similar results and the client shouldn't really see anything weird if it's binding to still version 1 and not version 2. With version 2 it would require to wait for the respective events.
See also https://gerrit.automotivelinux.org/gerrit/c/apps/homescreen/+/27860 which is the implementation for the qt homescreen which supports version 2. But it works just fine with qt homecreen not being updated.