In T53084#465872, @Lapineige wrote:

@ashley (watercolor): this add-on was first intended to be used from object mode (which is the most common case), then I allowed edit mode but probably never tested that case of undoing inside edit mode. That's why I didn't see this problem.
Until this is fixed, if you have global undo activated, swap to object mode and then use AutoMirror. Then you should be able to undo it (from object mode) without undoing previous changes you made.