Inventory items on hand error

I am having issues with my "items on hand" in my inventory.

This is the second item I have come across today with the same issue.

I have attached screenshots for reference. The purchase transaction shows the correct initial quantity as "30" but in the same line the on hand quantity has already been reduced by 11.9? The two sales invoices where stock was allocated reduces the remaining amount accordingly, but there is still 11.9 unaccounted for. Usually I would expect all transactions affecting the stock item to be included in the list so you can track where stock has been allocated, but there is nothing else showing where the stock could have gone.

The current value of the stock in the items register also differs from the ending balance of the item when you search for the item - the "find transactions" ending balance is inclusive of the value of the missing 11.9 whereas the items register is not. 

Can someone please explain to me what happened to the missing stock? Is there something I did wrong, or something I am not seeing?

For the first inventory item I found with this problem, I manually adjusted the stock on hand value via adjustment journal to put the missing stock back into the items register, but if this is something that has affected multiple items I would like to find cause of the problem rather than manually adjusting each item individually as I find them.

Thanks