Rounded Hype objects create square-cornered masks.
The easiest method is to create the beaker and outer white mask as a PNG with a transparent center, then center it on the liquid layer, select both and group them, then set the overflow to hidden. The white mask would clearly only work on a white background.
Hype objects are not true vector objects in the same way as they are in Illustrator, After Effects, etc.
A while back I created a "machine" that filled paint cans, with the can having a transparent oval at the top and an oval-shaped colored object rising behind this to look as though the can were filling. Behind this was an object with a fill to represent the interior of the can.