the banjo disappears and changes to piano.
There's not much information here to go on. My best guess: That channel was reset with a Program Change message of 1 at the beginning of the .mid file. Insert an appropriate Program Change message there. It's impossible for me to say exactly what message, but if I assume a General MIDI setup, I'd suggest a Program Change number of 106.
One special note: Some synths consider the range of Program Changes to be 1-128. Others use a range of 0-127. If that's the case, the piano would be Program Change 0 under GM, and the banjo would be 105. You end up having to subtract 1 from the 1-128 convention.