Correct material transaction - Transaction quantity is more than existing transaction quantity

Batch has a transacted qty - Transaction type is WIP issue.

Ingredient was not user and returned using a WIP return transaction.

We then attemtpted to make a correction to the WIP return using the 'Correct Material Transaction' form from Batch details.

Error on saving: Transaction quantity is more than existing transaction quantity

All qty's look fine.

Anybody see this before. I've even managed to reproduce on non prod but no idea what's causing it.

thanks

Joanne