This is it!
Repro steps:
1) Enter Combat
2) Cast Concelhaut's Parasitic Staff
3) Finish Combat
4) Save while the temporary weapon buff is still in effect
5) Load the game
6) When the temporary weapon buff wears off, your old weapon&grimoire will be gone for good and those weapon slots bugged.
The issue is that saving when you're missing your weapon and grimoire due to the temporary buff will not save the items that have been removed, so when loading the data is lost for good.