terms auto-multiplied

Terms are automatically multiplied when I delete one. Please watch this:

And now step by step:

1. As you can see on the image below (this is the Termbase Management window) there are 743 terms in English, 405 in Italian and 347 in Polish.

2. As I open the Terms view we can see one term is already multiplied.

3. I open the view of that particular term ("Alpi Carniche") and scroll down to view the related terms in the Polish language. You can see there is a lot of multiplied terms in English (and Italian) and there is a duplicated term in Polish. (Believe me, I might have made a mistake and added a term twice but for sure I did not multiply them myself!)

4. I delete one of the Polish terms. Everything seems to go well till this point - I get the information the term was deleted and the changes saved (sure it takes time with this number of terms for a single entry!).

5. Then I switch the view to another term (all is fine). When return to the previously viewed term ("Alpi Carniche") however, I can see the duplicated term in Polish is still there!

  

6. Now, look at the Termbase Management window: 1127 terms in English (compared to 743 before the term removal attempt), 533 in Italian (405 earlier) and in Polish there are still 743 terms!!! (Please note, the number of entries did not change.)

Can anyone explain what happens here and how to stop this?

(Edit: I use SDL Multiterm 2021 SR1 16.1.1.541 @ Win 10.)