When I examined the 1Password entry, I saw it only had a password and not an account name. Adding the account name caused it to fill into Discord correctly.
I think this is probably Discord's bug, and they're probably handling the null user account poorly. But you might want to look at it, too, and make sure. Missing account names are probably not the usual path. (You also might be able to fix this by returning an empty string instead of null.)
1Password Version: 7.1 (70100006)
Extension Version: n/a
OS Version: iOS 13 db 4
Sync Type: 1Password