The rules I was using for grenades are that you roll vs. AC 10 to hit the square you want. Then if you miss, you roll a d6/2 and a d8 for distance and direction of the miss. Then it does the thing in that square, depending on size, etc.
The dodge action applies disadvantage against any attack on you which I didn't make - the floor wasn't dodging. Attacking from a hidden position gives you advantage on any attack roll, he was hidden so advantage. Though, it's dumb because an inanimate object like "the floor" probably should always give you advantage against it.
Attacking inanimate objects makes shit all fiction-blurry. D&D isn't perfect, so we go with what makes sense in the moment. We can't stop and debate forever, you know? Make due with what we get. I'll probably ponder that v specific scenario and make a permanent ruling.