Then you did something wrong - maybe got a save with ALMOST everything unlocked? I remember only being able to watch it after getting the 100% save from here; had everything else unlocked in my playthrough, finished the game, and was trying to get that last "Memory" (Mudman 2)...
But it was an apparently VERY rare occurrence (RNG), getting caught by the monster while hiding in the locker... Tried for like a good 80 minutes, but decided it wasn't worth my sanity, so just got the 100% save from the thread.
So it's in here somewhere.