User priv strange behavior

I’m using RT 2.0.15 with MySql Ver 11.18 Distrib
3.23.54

Yesterday I had a user complain that their options
after logging in looked different. I verified and
found that somehow their rights had been removed. The
checkbox on the:

“Let this user be granted rights”

was gone. I checked the box, saved, and he was fine.
Today, I had another user with the same issue.
I searched all my users and found several with the
same issue.
Is there some process that automatically removes
rights with inactivity? Or is something else
responsible?
There are users that I know for sure have not accessed
RT in a while, and they were not affected. So the
"inactivity" theory isn’t looking too good.

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