Earlier quoted context omitted.
You're anthropomorphizing emergent behavior from endlessly generating billions of tokens on a task that's impossible to solve. Agents stop following instructions as the context grows even at the best of times. Eventually something is bound to go off the rails and it just snowballs from there.
It wasn’t one agent forgetting things because of context, they explicitly discussed with each other and themselves the problems with going outside of the parameters of the task.
The existence of that improvised message board just becomes part of the context, the same one where all the other instructions live.