Many thanks for all your replies!!!
You put me on the right track it must have something to do with DLNA and my QNAP NAS (I forgot to mention that the .flac files are stored on a NAS),
cause the location of these additional .wav files in JRiver points to exactly this NAS.
For example: http://<IP of my NAS>:8200/Transcode/A0$17432836.wav
8200 is the port of the DLNA-Media Server on the QNAP.
I deleted again all of the library entries in JRiver, imported the .flac files and listen to them via my Samsung TV. This does not have an effect.
I still see only .flac files in JRiver. So I think QNAP does some transcoding by itself independently from the Samsung. And JRiver imported
these transcoded files automatically in the library.
Since I want to keep the Auto-Import function in JRiver, I have to find a way to stop QNAP from transcoding my files.
By the way - in the Auto-import settings I have only included one folder with my .flac files. No temp or subfolder visible. I guess /transcode
is a hidden subfolder.