I was holding my work belt which had my hammer, shield, and I think shears in it. I also pressed b on accident while holding it, hovered over my shield to take it out and then it switched into the belt slot making my belt and its contents disappear.

I think making it so players can't press b and also shift+b while holding a belt should fix it, or maybe just not being a dunce like myself.

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This was fixed in 3.4.0.