Signs get deleted by composters when throwing signs at the composter.
When you try to place signs,it looks like it only has 4 items,but actually has 2 more items(invisible)leading to signs being placed with fists

How to reproduce:
1.Throw signs at composter(make sure to leave some signs)
2.Place signs
3.For example if there were 5 signs,and you threw 2,the 3 signs will be placed normally,while the other 2 will be placed using hands