Oh man, I forgot this one. As you can see on the video, I can't find the last graffity spot. Maybe it's so hardly hidden and I just can't find it? If not, so that's a definitely the bug!
It's possible I made a spot invisible or inaccessible without noticing. I'll see.
e: Just checked all the spots. They're all accessible. There's a possibility you fell victim to a rare bug where a level 1 graffiti doesn't register as done.
I found the bug. There are in fact 126 graffiti spots, one is counted twice because it's made up of two linked spots. I know exactly which one it is, too.