Looks like you are onto something 
It is an issue with removing indexes in forward order.
For example if you try and remove more than 1 at a time… let’s say 2.
It will remove slot 40 first then it will try and remove slot 41 (but 41 is now 40 so it won’t remove it).
I just need to make it reverse the loop when it removes the slots so that it removes 41 than 40.
FIXED
I just came up with a great way to reverse loops in blueprint.
So you will need to create a Local Int Array*(I called it LocalAmountIndexes)*
in the InventoryComponent->DecreaseInventorySize() & InventoryManagerComponent->DecreaseInventorySlots() functions.
Then add the changes shown below. This will remove the slots & UI slots in reverse order and fix the bug.
I will be adding this change to the 1.3 update. Since it’s the first fix in the 1.3 update, the update won’t be released until there are a few more changes/fixes made.