I tried to split up group of object, nothing happen. I was on creative mode.
Seems creative don't allow you to "split up" group of items either in inventory or hotbar. Can be annoying, mostly in the hotbar where you may just need 1 or 2 items of severals group of 5 ; scrolling from number 1 to 10 can reach 4 turns of mouse wheel.
If it's not a bug but a restriction it should be nice to implement the system in order to allow at least the split in the hotbar but keeping it restricted in the inventory.
But in anyways you must warn the player of that restriction by a warning message (like for too short or too long faction name) or prevent him to reach the useless split button.