As mentioned. The problem is the scaling factor which may have been changed by the system itself. If there is a scaling factor also in 1Password itself, it perhaps needs to be removed to re-fit the system scaling factor. This is done possibly by the system (or KDE) for the purpose of unifying the scaling making a better UX.
It shall be noted that the Brave Browser also had this problem since the update of Kubuntu 21.04 but quickly had it fixed. So far, not yet for 1Password.
1Password Version: 8.1.0-45
Extension Version: Not Provided
OS Version: Kubuntu 21.04
Sync Type: Not Provided