Iron Chests for Fabric's chests can't be rendered as static blocks, which is what FastChest does. It looks like Minecraft tries to render the chest both as a dynamic models and as a static one. I'm not sure which mod is responsible.

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I fixed this on the FastChest side now, to make them rotate properly I'll also have to get some changes to the block state jsons merged into Iron Chests.

4nner/IronChests#20