I updated my packages today on my FreeBSD 11.2-p1 system with Plasma 5. plasma5-user-manager was a new package, and I decided to see if it works well on FreeBSD. I tried making some changes to my own and other user accounts (simple things like changing the icon). When it asked for my password, it seemed to accept my password (no error message) and then immediately ask for my password again. It will ask for my password 4 times each time I try to make any changes. If I input my password incorrectly, it will give an error message. But even after inputting my password 4 times, it apparently never gets authorization, and no changes are actually made to the accounts.
Given that you have _2, I assume you already have the dependency on accounts services (sysutils/accountsservice).
I'll look into it.
This problem exists on both Linux and FreeBSD. It is also present in 5.14.2. It appears to be an upstream KDE Plasma 5 bug rather than a FreeBSD-specific bug.