Ooooh. That may well be a challenge that behaves differently around people who were or weren't supposed to be there.

Perhaps solving the logic puzzle is sufficient to gain entry, and most students will either not think to flip the parchment over or get bored before they finish brewing the potion it describes. But Snape expects Voldemort to notice the lack of wards and decide to brew the potion, and accordingly has booby-trapped the potion in some manner that triggers upon its completion or after some time sufficient that he expects students to get bored first.

The duration may well also be significant for Time-Turning.