[itsme]

Yes, that's it. I was mislead by starting key erroneously set to C/Am whereas the ChordPro file in fact was in Bb.
The "#" and "b" buttons only change the sequence of destination keys. The usage of flats and sharps within the song depends on the selected destination key. That's correct.
To use sharps in case of C/Am as destination key is also OK.
An option to change the usage of flats and sharps in the song independent of the destination key would not make anything better, but would be an endless cause of confusion.
But "Detect Key By" should be used on import to set the starting key and the "Keys:" meta tag in case no {key: tag is found that overwrites it. I have no doubt that this is a bug (or a missing implementation).