Here we are, months later. Nobody has figured out how to get this damn thing.
- There's supposedly a requirement of 200 Happiness according to Serebii and sites parroting them. I've had Pokemon gain it that were barely at or over the limit, but there's at least one other requirement because I have dozens of 'mons at full Happiness who haven't gotten it.
- Nothing to do with walking or Let's Go mode. Tested and confirmed, despite "common knowledge" on Twitter, I've had 'mons get it without having ever taken a step outside their ball.
- Nothing to do with raids. Again, tested and confirmed, only one of my raid Pokemon have it and most of the others who do have never even seen a raid.
- Happiness and Friendship are a single mechanic and have been for several generations now.
- EVERYBODY has a story about "well, I have a Pokemon with the mark and I did this, so..." that doesn't actually help because there's no evidence.
- One of my dedicated 'mons who I've kept on my team almost the entire game doesn't have it. One I bred and fed candies to then spent 5 minutes EV training does.
It might legitimately just be random.