I'm on the newest AA version 2.10.2. FiiO just did a major update and I'm on their latest too.
Volume between my Axiom Air and the FiiO is the same as before the updates of my FiiO and AA. I have bluetooth headphones and speakers that work fine with the FiiO. Just my AA that has issues. I am happy to do further testing/troubleshooting. Just let me know what I can do.
I use the FiiO quite regularity over wifi with my AA and volume works as expected so it the issue is only over bluetooth.
I ordered a new dongle to test but I messed up and didn't get one with 4.0. I may order yet another to try.