Seems to be a bug. To recreate just use a composter with an empty hand and it fills up like an item is being used on it.

Tried on Cultural Delights 1.0.4 & 1.0.5 (1.20.1)

(Translation used)
This bug has been found since version 1.0.3 and is still present in the latest version. We need a way to fix this.

hopefully this gets fixed soon, it's a very strange bug and it makes it very difficult to use composters without accidentally getting extra free bone meal, hoppers being the only consistent option.