Eh! The problem is that a bug exist, it has been there a while and it will be a bear to uncover without some effort, which is why I volunteered to run a debug version.
For context, I uncovered it a long time ago (many generations) but since Mp3tag was 30th app installed, I could not rule out my system as the issue. So I tolerated it. Now, I have tested it on 4 machines to the same effect. I have tested a completely clean Mp3tag install on all 4 to the same effect. I have tested the same on a new segmented PC (no network, one other app-not running, up to date W10, ample memory, SSD, fast processor) to get the same result.
Caveat: 1) The only time it works as intended, this portable install, is when run temporarily from a Linux-Mint boot USB drive, installed with Wine and run on the same rig. (There are other Wine/Linux related issues or I would just use that and withdraw the bug report. 
2) Rarely, maybe 1 out of 100 times, the second row selected stays put. The next added row always cause the jump to first column.
I am unaware of Florian's development and validation process, but feedback from a forum is a limited sample size and may not be even diverse enough. Folks just tolerate some things since Florian has already contributed so much (a useful and current free product).
While it may be a caused by another application stealing focus, I have the issue on a new system. Unless the culprit is Windows… (Ha!) I would look elsewhere though.
Sorry to respond to the DE group in EN.