Playing a 3 note chord on GarageBand performs an UNDO function I have a 1st edition iPad Pro 12.9" and I opened GarageBand. It loaded an old song automatically, and the instrument was a synth. I switched it to a Grand Piano and began to play a 3 note chord. As I played the chord a couple of times, the instrument switch back to the synth.

I tried a few more times and the same thing happened. I finally tracked it down to it performing an undo function, and I was able to select redo to change the instrument back to a Grand Piano. Every time I play a 3 note chord, it does this undo making it very hard to play what I am trying to play. It happens for any chord I play and on both sides of the screen.

I am running the beta of iPadOS as well, if that matters!

Is this normal? I've not really used GarageBand on my iPad much...
In iPadOS 13 undo and redo are three finger gestures. Undo is supposed to be a three finger swipe to the left and redo a three finger swipe to the right. The movement to or from a three finger chord certainly may be recognized as a quick swipe. Try disabling gestures in Settings. Because this is in beta the location of settings like this may change between releases. I'd look in Settings->General and disable gestures to start.