I always find if I press ctrl+z it’s very inconsistent. Especially in the sampler track. If you put a file in the sampler track make some changes and then want to go a step back by hiring ctrl+z, the whole sample is gone with all changes. Very annoying. I like the way the Reason DAW does it. If you hit ctrl. Z you get a step back and it shows you where you last edit was by zooming or going to the last specific point. Very convenient. 
Hi and welcome to the forum,
The Sampler Track “editor” is an instrument in fact. Therefore Undo function doesn’t undo what you did in the Sampler Track “instrument”.
1 Like
Also keep in mind that the Sampler Track doesn’t actually have undo steps, so Ctrl + Z doesn’t work with it at all anyway.
All of the Sampler Track operations are non-destructive though, so you can always go back to your original sample no matter what.
Hi,
That’s because it’s an instrument in fact. Same as you don’t have an Undo for the changes in the plug-ins or the instruments, you don’t have Undo for the Sampler Track.
Well, to be exact there some of the most used plugins out there, which have undo/redo operations, like Fabfilter, Waves and Slate Digital.