This is due to Forge's vanilla bucket capability restricting itself to stack size = 1. In practice, this is necessary, as otherwise you expose the ability for a fluid handler to get a stack size > 1, and start interacting with it, which can cause all sorts of problems. But it means that everything that wants to interact with a fluid handler, first has to copy with stack size = 1, then modify, then adjust. Which is a total pain and requires redesigns in FluidHelpers#updateContainerItem.

The exposure of this bug is, with a stack of vanilla buckets, interact with typical fluid handlers: right click on filled barrels, cows, etc.

Another possible bandaid fix is to make the vanilla bucket stack to one, just like other buckets in TFC. There are also other consistency issues between vanilla buckets + TFC buckets, it may make sense to replace it outright? Or some other solution.

Fixed by 41d1923