No one should be slipping out if they meet the height requirement. These restraints are used on Velocicoaster and if someone could slip out, they'd never have installed that zero-g stall. You need to be stapled into the seat to go with a lap bar on an inverting coaster. Only caveat would be if, in addition to be wheelchair-bound (which shouldn't really impact this as long as the person has legs), if there were a serious cognitive deficit, a person could forcibly move into a dangerous position. I presume that's one of the few cases where Universal isn't liable here. But, there are no details suggesting this is the case.
Surprised this didn't happen on one of those RMC wooden-to-steel retrofits (i.e. Iron Gwazi) first. Those invert, have lap bar restraints, and include zero-g stalls. And they really beat the trap out of you. I find them to be much more aggressive thrill rides than this.