I wish I had an answer for you. We have not been able to reproduce any of these reports regarding PM notifications not going out.
My/our suspicion has always been that users may have some kind of overactive spam filtering or a “focused inbox” setting enabled that is “helping” in an unhelpful way.
Anecdotal but I get lots of PMs given my role here and I get notified of each one. There hasn’t ever been a surprise PM where I hadn’t received a notification. It’s definitely not happening to 80% of my PMs.
Perhaps relevant is that I don’t use Gmail/Outlook/etc for my personal email that PM notifications go to. It’s hosted independently. I do use spam filtering but I don’t use any focused inbox helpers.