I had that with my bt speaker. What helped is to get both devices to 100% when the music is playing. That guaranteed that both devices were on their actual maximum
I did go into developer settings last night and disabled the audio sync. "Disable absolute volume" and that didn't seem to change anything. (I did not try to raise the volume on my buds after I was at max on phone though)
I've never noticed this 'issue' before. However I realize I did just copy a bunch of crap to this phone, install updates, etc... I'm also having some weird tasker behavior where it is randomly crashing. That was actually the driver of the reboot this morning as I had updated/toggled some permissions that didn't seem to be sticking.
I only toggled it and saw no difference so I turned it back off. So far, the morning reboot fixed it, but I'll be monitoring it closely. I use my headphones a lot. Also Not sure how to turn 'off' the pixel buds. I'm sure there's a way to reboot them I'd just need to search.