This whole post is speculation, based on things I've witnessed with AC lounge entry, but I could easily see it being an implementation issue.

As far as I know, the MLL entry scanners only look at the BP. BP's are signed, so you probably can't just make your own and use it to get in. But they look at the BP data.

And if I show up at check-in at 1600 for a 1700 flight, but the flight has been delayed to 2100, my BP says 2100. And now the MLL system thinks I'm there 5 hours before my flight, and not allowed in, even though per policy, I'm allowed in.

However, checking flight status should be pretty easy.

But I have a relevant Centurion Lounge (which has the same 3 hour rule) experience.

I was booked on an AC SFO-YYZ flight. It was delayed something like 5 hours. So AC rebooked me (without asking) on a (now) earlier SFO-YYZ flight.

That was not a fun lounge check-in experience. My (current) flight wasn't delayed, and I was there 4 hours before departure. I had to dig through emails to prove I was originally on the other flight that had a posted delay.

Imagine you had a UA ticketed/operated flight in that scenario. AC can't pull up your PNR (maybe they can pull up the ticket history*). How the heck is the system going to know you're allowed in? Do you really want agents to be reading passengers' emails in that scenario?

*But was that a voluntary flight change?

Similar to "we're going to switch over to dollar-based points earning in late 2021", it's something that sounds relatively simple, but becomes exceptionally complicated when you start dealing with partner bookings and IRROPs.