[itsme]

We had lengthy discussions about this feature and I was involved in many of them. Nevertheless it confused me again a while ago, so that I had to think about and search the forum how it is intended to work. This is what I found:
The "b" / "#" buttons only modify the base key when transposing is applied. When "b" is selected, transposing a semi-tone up from the key of C the result is Db, when "#" is selected it is C#.
The buttons  do NOT influence the accidentals of individual chords within the song. Those are only depending on the base key: in "#"-keys like G, D, ... / Em, Bm ... the chords are displayed with #s, in "b"-keys like F, Bb, ... or Dm, Gm, ... the chords are displayed with bs.
For the key of C the decision was to use #s.
That's how it should work. Not perfect in some special cases, but usable.

When verifying my sentences above I noticed a small bug:
Slash chords like C/G are transposed correctly, including the bass tone after the slash. This is not the case if "0" is used instead of "dim" e.g. C0/F#. It seems that "0" is not regarded as part of a regular chord so that it interrupts chord detection and the following bass note is not transposed.