Tested under BG2EE (v1.2.2030), but also true for BG1EE (v1.2).
Observed behaviour; When you use a potion or use an item to give you a +STAT (or a spell) allowing use to equip an item normally impossible to use by you (or by a specific NPC) -ex:bow, wand..-, when the potion/item/spell effect disappears, the weapon/wand/item/etc's slot is reddened under the inventory again..but not in the general UI window..so you can always use the item in the general window if you don't remove the item in the inventory (ex; bows, wands..)...
Expected behaviour; Well..you drink your potion/use your bracers/spell(etc), you can use the item... but when the +STATs effect disappear, you can no more use the item under the central window UI (the icon disappears OR be reddened like in the inventory..and the action blocked).
[ex; tested again with BG2EE v122030 with Valygar and a bow normally requiring STR18 for exemple -that last bug is also always existing under BG1EE v12 btw - under BG1EE; Yeslick with a 'genius potion' can put a 'wand of heaven' in a quick slot..and always use it after the effects of the potion disappear.., etc.].