If I will add key signature to my tablature (Composition - Key Signature - 1 Sharp) and then save the file and open it again the key signature does not exists anymore ... is this a known error?
Hi, that's not an error.
that's because you are saving a file in a fileformat not 100% compat with tuxguitar. ( tuxguitar support key signatures peer measures.. but GP3 and GP4 has support only for tracks)
try save the file in .tg format.