Note: This is not an SBC issue and both the Spotify app and Finder app can exhibit this problem.
Sometimes when I connect my Sony WH-1000XM3 bluetooth headphones to my Mac they are fairly quiet at maximum volume. When I change volume right after the connection is made I sometimes see the volume suddenly jump to what appears to be the volume remembered by another "profile". I'm usually able to reproduce this by turning the headphones off and back on again. I did some research and learned about the different bluetooth profiles and codecs that the headphones support. Based on that I initially thought that what was happening was the headphones were connecting using HSP or HFP instead of AVRCP or A2DP. This is not the case as I have confirmed that even when connected using the "low volume profile" the codec being used is AAC, which is not supported by HSP or HFP. The only "fix" I've found is disconnecting and reconnecting the headphones. That always seems to put them into the "normal volume profile". If I then switch them off and back on they will most of the time reconnect in the "low volume profile". Anyone know what is going on, and how to properly fix it? I'm on an early 2015 13-inch Retina MacBook Pro running macOS 10.15.7.