Stack size splitting loses half the stack

When i split a stack (modified using the stack size manager) it removes half of the items in my inventory.

Example Stone:
I have 2k stone in my inventory i split it to 1k and 1k

1k stone will be moved fine

The other will despawn 999 stones so 1 will be left in my inventory.

I dont use any other stacksize managers as i went to this setup as its a great all in one package.

Example below shows what happend to a stack of 5 on bean cans.

This is 100% a conflict with another plugin