It's meant to be much more risky. Entering sand is almost always dangerous. When you're in shallow sand, all adjacent sand is safe. If you're in deep sand, there might be quicksand nearby.
No I get that, but getting screwed over by bad RNG where a level is straight up unbetable is just plain annoying and stupid. I think it could be mitigated by maybe making it so if another character is standing inside of the selected girls adjacent tiles would prevent them from instantly sinking next move. Or make it a timed based thing idk.