iPhone iOS update switched passcode from 4 to 6 digits

My mom recently switched from iPhone 7 to iPhone 13. I set the phone up for her and at the time, just 3 weeks ago, it allowed her to keep her 4 digit passcode, along with all of her other settings etc. I thought it was a bit strange that it didn’t force her to update to a 6 digit passcode, but didn’t worry about it because it would be easier for her to remember her old 4 digit passcode. Today she called me and said her phone now requires 6 digits for her to unlock her phone…. However, after what I assume is a software update…. She wasn’t asked to set up a 6 digit passcode, but she now has one and we can’t get into the phone. Is this a glitch? Is there a default 6 digit passcode we can use? Is there anyway for us to get into the phone without restoring it? As it will be incredibly difficult for me to do that or walk her through that process of recovery mode/ restoring.

Thank you in advance.