TL;DR: click the circle “i” icon in the Bluetooth menu to go to device information and select the device type as “speaker”.
I recently ran into an issues when trying to play music on a Bluetooth speaker where the volume was super low. If it turned it down and then back up I would it hear it go up, but then it would immediately seem like the volume was turned back down.
During my troubleshooting I check all the usual suspects, I reset the device, restarted the phone and then tried another Bluetooth device. After trying the second device with the same issue I knew it had to be the phone. Interestingly when I used a second phone I got the same result.
It was then that I thought about what Bluetooth settings the two had in common. The only one I could think of that would adjust safety being Apple’s “headphone safety” feature. To test it I turned it off and BAM! The volume worked. At that point I just had to figure out how to keep it on AND use the speaker. After checking the info section for Bluetooth by clicking the circled “i” in the device setting section I found I could set the device type. I set it “speaker”, disconnected as reconnected the device and now I get full volume without having to turn off volume limiting for regular headphones!