Same here - I had never even heard of anyone leaving early from Church until I started attending Mass.
I don’t even remember being told anything about it - it’s just not something that would have ever occurred to anybody in my Protestant church.
Arriving late was also extremely rare, and usually involved people who were arriving by bus - the usual reason was, “The bus was late/ the bus didn’t come and I had to call for a taxi.” We would sneak these people into the side aisle and hope that neither of the ministers noticed them being late.
But yes, even when we were very little, I remember my Mom getting us ready for Church an hour early, so that if anything happened, we’d still be on time for Church. Nothing was more important than that, and it didn’t occur to any of us that it could be any other way - it was more thinkable to be late for school or for dinner than to be late for Church (in the sense that these were things that could enter your mind and then be dismissed as silly - being late for Church didn’t even enter our minds at all).