Oh sure, the wound may have been fatal. That is not in question, just that it didn't drop immediately. Hitting it with more shots was the right thing to do, however, because if you don't connect with the spine or a major blood vessel (which you can miss behind the ear) and hydrostatic shock doesn't work its magic, you can be left with a simple flesh wound through big neck muscle.
You mentioned the mud camo above. I have seen coons come out of my water hole nearly invisible on thermal except for their heads. I like this video because this guy has a similar issue...
IIRC correctly, he has a driveway alarm on the feeder to alert him in his house some 80 yards away that animals are under his feeder. When the alarm goes off, he goes after the hogs. So he goes after them with his rifle and thermal scope. By :45 in the video, you can see some of your camo'd pigs. Either they are wet or well mudded. The patterns vary but it looks like they have been partially submerged.
At 4:20 in the video, something funny happens that is camo-related. The last hog crosses the FOV and the hunter takes a bead on it when it stops. He fires and the hog runs off, but you can see where the guy shot based on the glowing spot of the feeder leg. The feeder leg has become nearly invisible to thermal because it is ambient temperature...until it gets shot, LOL.