This is one odd bug(as a bug it must be).
I was trying to help Perodua Man with his server when he told me he was having issues with it.
The weird thing was that when we added me as admin, my access level stayed over reconnects and mapchanges, and everything worked just fine.
But when we added him(Perodua Man) as superadmin, his admin level was lost every time we changed map/he reconnected, and we tried everything, including removing the data\ulib\users.txt file, setting the users.txt up manually, completely redownloading ulx and ulib, removing all other addons, and lua scripts, removing everything in the data dir, and so on.
Nothing would fix it, oddly enough.
Now a few weeks later, I'm experiencing the same problems on my server
I've have been running it with ulx svn since forever, and so I have a set of admins(and myself as superadmin), but since some svn update, no players I add(ingame or manually through file) keep their admin status after reconnect, or mapchanges.
Though all older admins I have keeps their status(users.txt file corruption perhaps?).
I've also tested this, by joining the server with another account of mine, then running rcon ulx adduser freefry admin 0, which adds me as an admin without immunity just fine(I can use all admin features for that access level), BUT when I reconnect, my access is lost, and I'm reverted back to a normal user. (same if the server is restarted/changing map).
Now, I've noticed one... anomally should we call it, and it resides within the data\ulib\users.txt file, which I have attached.
The anomally is with users with 'special' characters in their names, as for example with my steam name, it contains a " character, and ulib/ulx seems to readd my entry, forever, with alot of \\ infront of the " character.