Can you post a screen shot showing "the ♯ (not ♭) render with a grayish colour that differ from the chord-letter and is near invisible"? I don't see that problem.
I do see that the symbols aren't handled correctly with the chord coloring and ChordPro positioning in the web interface, but I don't see any problems in the Android app.
BTW, if you change Settings > Appearance > Lyrics Font to Monospace, the app will use normal b and # characters instead of the special characters, to ensure consistent character spacing.