After clicking the "Add New User" button there is no obvious way to cancel this operation. I know to click on an existing user but this is not immediately obvious.
Yeah, can confirm.
*** Bug 459699 has been marked as a duplicate of this bug. ***
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/5658
Fixed on master should be available in 6.5