As reported here, long holding on a buff icon to open the details popup sometimes does not work. This was reported on Pixel 7a / Android 15.
I actually noticed this myself too the other day, but then later I tried again and long press did work, so I assumed I was just mistaken before. This was on Pixel 8 / Android 16.
Now that someone else has reported it (and commented "sometimes"), it seems there is some sort of bug after all. Please reply to this bug thread if you've observed this as well. If you have any details about the conditions under which it happens, that would help a lot too. Right now I am not able to reproduce the bug, which makes fixing it challenging.