This sounds exactly like my very old problem that started happening to me for an unknown reason
Although my outcome is "only" similar [as it is my Filter that gets constantly moved below the toolbar with icons] but the triggering of this bug is exactly the same [in form of re-opening of Mp3tag]
But there is hope. Because also for whatever reason, on my new PC on which I installed everything from scratch, the frequency of this problem occurring have dropped significantly. Which does not make sense because this is my second installation [from scratch] of Windows 10 - although maybe you could try [just like I did] to install the version of Windows 10 from January of 2021? And install on it Mp3tag version from that time?