Ah o.k., it does make a difference. I now can repeat the behavior of the player, which I considered an error.
Til now I have only used the search to look for what is buried in the 'safe' or to add found items to the playlist. I've never used the 'apply' button and wasn't aware, what it exactly does in combination with 'skip non-matches'.
What had happened? I had searched for an artist or a release, to see, what exactly I have. The results were found in the library, none of the search results were in the playlist and I didn't add them to it. Befor closing the search dialogue I must have accidently clicked 'apply'. That click remained unvisible, because the applied tracks weren't in the playlist. The list contained only 'non-matches', and thus the - for me - irritating behavior.
The command 'apply' remains active, also when adding or deleting items to/from the playlist or closing and reopening the app. This I find a bit weird. The only way to delete that command seems to be to open the search dialogue, clear the search field and then click 'apply' again. Of course, the solution in my case also would have been to untick 'skip non-matches'. Now I know...
Thank you very much for your help and effort, to shed light on this. Good to know that nothing mysterious is going on here.