I have many songs that have some preset volume levels that change throughout the song. Some start at 0 and go up slowly for a swell effect and back down again, etc.
On many of these midi files, if I adjust any of them in any way using sysex messages, doesn't even have to be the volume. Could be me changing an instrument, transposing the key, whatever; they don't work properly anymore. Usually the volume just won't change anymore. If it starts at zero, it STAYS at zero and you don't hear the instrument. Even if I try to manually change it, it fights me and goes right back down. Very frustrating.