1. Enter 123123 into the TOTP setup screen
2. Focus on the first box.
3. Type 4
4. Auto-tabbed to the next input
5. Type 5. Unable to, until you delete the character in the field.

Ideally at step 4, upon tabbing to an input that has contents, it should select the existing content to allow typing over it, but not clear it.

This could also be achieved as a general "Upon field being focused, select the character" style to allow easy overwrite.