Just wanted to put this out there for anyone else who might be hitting the same problem and be searching the Web for a solution.
I have a Sennheiser Momentum 4 headset. It worked (reasonably) well most of the time, albeit with some occasional momentary connection losses and what I think was the occasional crash-leading-to-spontaneous powerdown)
However, in an attempt to resolve these, I recently used the Sennheiser Smart Control app to update the headset's firmware (note that the occasional powerdown still seems to occur).
This headset supports Bluetooth Multipoint functionality; it can be paired with multiple devices and used with them concurrently. I have it paired with an Android phone and a Linux laptop.
After this and a restart, I wasn't able to play music back on my Linux laptop. After poking around a bit, I discovered that I could get sound working if, in pavucontrol (PulseAudio's control panel), in the Configuration tab, I chose HSP/HFP. However, this also resulted in degraded audio. If I chose A2DP/aptX, then there were no apparent errors that I saw (and after a few reboots of the headset I did, somewhere along the line, the A2DP/aptX option didn't even show up in the menu once the headset was paired).
I'm fairly confident that this is the same problem that someone on Reddit experienced here, as it sounds identical: a Linux laptop user with his Momentum 4 headset also paired to an Android phone using Bluetooth Multipoint that stopped playing audio in A2DP/aptX mode subsequent to a firmware update).
Further investigation revealed that the headset will indeed play back audio from the Linux laptop in A2DP/aptX mode while concurrently paired with the Android phone, but only if the phone is not set, in the Android Bluetooth system settings, for the headset, to have "Media audio" enabled for it, just "Phone calls".
I definitely had played audio back prior to the firmware update on both the phone and laptop, and the headset is billed as supporting Bluetooth Multipoint, so disabling "Media audio" on the phone definitely isn't a fix. But since I rarely actually play media audio from the phone, it's a good-enough workaround from my standpoint to get the thing usable again. I certainly didn't want to lose media playback on the laptop.
Just a heads-up in case anyone else out there with a Sennheiser Momentum 4 using Bluetooth Multipoint smashes into similar problems, on the off chance that this is also a doable workaround for them.
I would have to add that this hasn't been an very satisfactory experience, for anyone else who might be considering purchasing a Momentum 4 for Bluetooth Multipoint use.