Notation: Key signation not working well for Display Transpose

Hi folks,
I want to report a bug which I observe since at least Version 6.5 and which has never been fixed so far; my latest Version in use is 9.5, but I assume, it’s still present in version 10.

So, I’m composing arrangements, which include tracks have to be played by real musicians, so I want to transcribe their parts into notation-sheets.
For example, I want to write notation sheets for Bb-trumpet. Therefore I would like to use the display-transpose-feature for the trumpet-tracks. The notes are shifted by plus 2 then, if I select trumpet, that is OK.

But I determine a problem with the signature tune: I select the corresponding key for the track (or the part of the track if the signature changes during the song). The signature key is changed by +2 as well, e.g. a D-key (2 #) changes to E-key (4 #) except a part / track is declared as C-major- Then of course the key should be switched to the D-major signature for the trumpet, but the signature-key stays in C, while every single F and C-Note in the notation gets it own # instead of the right key signature at the beginning of every sheet-line. (If I declare D-major the key is switched to E-major, no change to get the D-Major-signature key ever…)
This misbehavior is completly reproducable - My workaround is to renounce the display transposing feature, transpose all notes of the trumpet-tracks by +2 manually and select D-major as clef in that case.
Could someone confirm that?
Thanks and best regards,