Model resets when I do a fix and undo

When I do a change into a model I imported and undo, something weird happens. It goes back to how it was when I imported it (resetting, so I had undgrouped them, it goes back to being grouped and everything I did doesn’t change). So basically the recent change I made is undoed, it goes back to the original.

The photo below shows how I changed its position and scaled it. I then decided I didn’t want it scaled so I undo but it goes back to this

Please fix this. Thanks