Patrick,
What software version is running on the switches (show version)? I believe there are a few known AVB issues that specifically impact fiber ports. I would suggest upgrading to the latest software (currently EXOS 16.1.2.14) as it contains all available bug fixes and enhancements.
What is the port status and propagation delay of the fiber ports (show network-clock gptp port )? The port status should be "master" or "slave" and the propagation delay should be lower than the propagation delay threshold. If not, that could indicate a gPTP problem on the port (a common reason for AVB problems.)
Also, and based on your description I believe this has already been done, make sure you enable AVB globally and at the port level (enable avb, enable avb port )
To your question on vlan membership, yes all AVB endpoint ports should join the AVB VLAN (assuming they are all participating in a stream of course.) So, all AVB switch connections and endpoint connections should be present in the AVB VLAN. Some of the Bi-Amp equipment has a non-AVB MGMT port. These ports will not participate in AVB and you will not see them in the AVB VLAN nor will they be AVB capable ports.