This is the biggest issue with mirror indeed.
Happends often when hooking mirror when already logged in
Logging out and back in is usually already enough to fix it.
Or just randomly after a few hours or days, hard to tell what actually causes it, I did so many tests and debugging to trigger this effect.
Afaik it's an issues with loading item definitions, ID's always worked for me, but I refuse to use them.
And sometimes it's completely broken als it requires a complete client restart...
That's why I primarly still use stealth client, to combat these issues
If you use mirror, just learn to live with this issue, been pending for years now