You mean when you move your cursor away from Potions on the GUI? If so, this is normal. The default hotkey for opening the Belt is the tilde key (~). It acts as a toggle. Press it once to open the Belt. Press it a second time to close the Belt.
When you mouse-over your Potions, the Belt expands. As soon as you move your cursor away from the Potions, the Belt collapses.
Are you clicking Apply when you set your Options?
You might want to look this over: About Custom Key Bindings
(The thread is about Key Bindings, but it applies to all Game Options).