When I press and hold the box to transfer a stack between inventory and another loot-table item, the little green bar fills along the top. When it is full, I release - but only one block transfers.

I have only tested this against chests and furnaces.

This is not the case when transferring between inventory and the hot bar. This does not affect the block splitting function.

Have heard, from a few people in forum, that others are experiencing this, too.