Is there any indication of whether this only happens if the screen protector was present prior to training the fingerprint?

> After buying a £2.70 gel screen protector on eBay, Lisa Neilson found her left thumbprint, which was not registered, could unlock the phone.

This suggests that an attack of "put a malicious screen protector on phone to unlock" is possible. I'm curious whether there was any re-training after applying the protector.