So sorry to hear about this issue but thank you for bringing it to our attention.
I’d like to process this bug report right away but will need the some further details to do so:
Kindly let me know your device OS and which Vectornator version you’re currently running.
Please confirm whether undo/redo is set to Default or Optimized. This can be done by opening the effected file → Settings → Preferences → Document → Undo & Redo
If possible, please forward the file in question for testing.
I have logged this ticket and will be in touch as soon as I have any updates. Again, our sincerest apologies for this frustrating issue - we appreciate your patience while we resolve things!