It's not just Mason, what about the AR?
For complex reasons, the relationship between the referee and assistants is often ineffective. The AR's obedient relationship with the ref paralyses the assistant even when the referee is dropping a bollock
This submissive behaviour stems from a need for united officiating and match control, but it often end with calamities which seem inexplicable
I had an incident when running the line on Saturday. Defender and attacker slide in, just outside the far edge of the PA (from my perspective). Defender is very late and should have received a cast iron YC. Instead the referee gives the FK to the defending team and protestations ensue. Because it's not on my side, I'm duty bound to remain silent. We must accept the referee's mistake, because of the dynamics of refereeing a football match
Back to Mason, the AR could have intervened, but the psychology of the submissive relationship probably explains why he didn't