You absolutely cannot trip their logic unless you leave in the first 90 sec after the key was dropped and do it hundreds of times in a week. That’s the intent and no, they’re not going to tell us the exact thresholds so that griefers can game around that knowledge, but The Blue Post was pretty clear.
If you read those bold parts and think that means you (leaving 6-10 keys a week out of 50), I really think the paranoia simply has hold of you. I’m sorry if that’s the case as it likely kills your enjoyment of the game. Either you can trust and ignore this or if it’s constantly nagging at you, do something else besides M+ (or even play a different game). People were literally spam-joining groups and leaving as soon as the key dropped and the timer started; that’s actual griefing with clear intent to ruin someone else’s chance at fun (there’s zero chance to misinterpret that when it happens so frequently).
The only ones worrying about this are griefers that still want to grief without getting caught and those who are simply paranoid and distrusting of most authorities. If I knew you IRL I’d put $100 on the table that you will never get banned based on the criteria Blizzard described above.