After casting the spell and then returning to my character as normal, I had lost prepared spells as if my intelligence had been decreased. Can't offer much more than that to help out with the bug.
It's because your items become unequipped, so you lose all spell slots gained from those. It's always been this way.
Seems unnecessary to me when other spells that replace your form (arcane eye, polymorph self) do not cause this effect.
POlymorph spell removes spells gained from items.
I always figured it was a small way to balance the spell's use, so that when you used it the wizard would be slightly 'weakened' by having some spells stripped. Of course I never actually asked, so it could be unintentional.
Arcane Eye doesn't do this.
Be thankful you're not a sorcerer! It's even worse for them, because the spell loss effect is cumulative. It is possible (but probably a pain in the ass) to script it so that when polymorphing your items are not unequipped and instead you gain predetermined statistics with that form, but yes, by default it will take off all your equipment pieces so you will lose wizard/cleric/druid spells at the furthest right side of your spellbook (those are the spells gained from bonus slots) or as a sorcerer or bard you'll just lose total spells per day, which grows more and more each time as opposed to the one time loss of other classes.
The polymorph effect is actually a measure to mitigate a game bug which causes PC to be momentarily visible when they transition into a new area irrespective of what effects or stealth they currently have active. As you no doubt know well, the item issue is a necessary consequence of the engine's inbuilt "polymorph" effect.
Arcane Eye does not use the polymorph effect. This is because the transition stealth issue was not considered as serious a problem for the Arcane Eye spell.
It is possible that the null human form modifies intelligence, but the mechanism is buried in obscure 2das and I do not have the patience to go digging for it.