Rapier critical damage wrong

Rapier in D&D 5e is a 1d8 dmg weapon. A critical hit should therefore do an additional 1d8 damage. However when you add the rapier to a player's character sheet, it automatically puts the critical at 1d6 and does not allow it to be changed.

Hi Douglas D.,  Could you provide a screenshot? This doesn't match up with what I've seen.  Edit: I just realized what you might be seeing, there is a "faded" bit of text in the critical hit section that says "1d6", it is not actually there. It is an example.  If that box section is empty (as it is when there is faded text) then it defaults to the damage.