Also,when I closed the game and reloaded the save file, the item was missing, and I had a glitched inventory slot that displayed nothing but still counted as occupied.
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As you noted, this is a rare glitch that has impacted your inventory. I don't have a basis for tracking it down, though, unless you can supply any insight as to what happened immediately prior... or perhaps what the powerful weapon was exactly. Could be the weapon is glitched, or could something transpired in game that either corrupted the inventory or (more likely) corrupted the game's memory in general such that it impacts your inventory in this way.
It was a legendary weapon I took from one of those Stone Tiles that require a feat of strength. I don't remember its name, but I remember it was a 2 handed axe that doubled my dmg output. I think the glitch started to occur right after I picked up a potion. I don't remember its name either, but I think its name was glitched (?)
Sorry I can't help you more, I have no idea what could be causing this. But it was right after picking an item, that I'm pretty sure.