Can confirm and found the cause: Redundant check in the ItemFrameItem class:
Can confirm in 1.20.5
Either grindstones should be movable like crafting benches or they should have an inventory to interact with hoppers like furnaces do.
Reproduced in 1.19.3
dreamcatcher3677
Can confirm and found the cause: Redundant check in the ItemFrameItem class:
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