Masked inputs – to clean or not to clean?

What are the arguments, from the UX perspective, to clean or not to clean masked inputs?

Consider credit card validity:

_ _ / _ _

Partially filling that field makes no sense, the partial value has no business meaning, and might be technically difficult to handle, especially if you store it in the background as year and month.

1 _ / _ _ represents no year, and invalid month (since the 2nd character is unknown, it can't be converted into number)

The techs would like to clean up the partially filled masked inputs on blur, arguing that it's not only technical problematic, but such values look weird for the user, and it's exactly how a date control is working - either you fill the date fully, or you get nothing.

On the other side, it might be annoying to have full value reseted only because you've forgotten the last character, and it's inconsistent with behaviour for wrong inputs (like 15/20), which are accompanied with validation errors.

Which one is better from the UX perspective of an average web user?