Not saying this behavior is set in stone, but I checked it out to see what happened since solid blocks will stop spit (and they do here), but from what it looks like the cops were at a slight angle which gave them a trajectory through the grates. The splash effect lands on the solid blocks since it's a pretty non-flexible effect so going off of where it landed it does look like it went through the block, but it actually arced over the edge since at that moment the cops were visible under the grate. I snapped a screenshot of your clip, take a look:
Here's my case (the circled cop looking at me was the one that hit me, where I was standing (on the spit pile) I couldn't see him because he was just barely out from under the solid blocks, but you can see how it arced over the edge a bit.
IMO if all it took was standing on 1 blocks above the floor and zombie cops were under you, you'd never get hit by spit until the block breaks. Their intention is to hit the player when no other melee zombies can while trying to take you down from your high spot. It could use some polishing, though, for sure. Just figured I'd explain it since I saw it while making sure it wasn't a glaring bug where they shoot through a solid block.