Yep I thought that too, the “re-read old song” way should actually be fool-proof. A rather complex app I was involved with development-wise (Blender, 3D software) does exactly the same thing, and it works without a glitch there. Although it has a seperate undo stack for certain modes, which only saves the relevant data for those. Could be applied to Cubasis and the Piano roll editor as well.
But I’m sure the devs know and are working on it already 