[DeArmin]

That's not a bug in MobileSheets, it's probably a matter of your chord localization settings. Your source file is in "default" syntax (Bb / B / C). In "Scandinavian" (Bb / H / C) Bm is not a valid chord, so it is not transposed.
With source localization = default and display localization = scandinavian your file is displayed and transposed as expected.

If you do the tranposing in the library screen it works. If you do this via Edit File (Datei bearbeiten), the transpose will not be carried out correctly (see atachment)