My accounts are 9 months old and I am only joining 5 groups daily, accounts actions on , comments on , I DO NOT post to any group
YET
Some still get photo ID .........LOL
My guess is: Facebook KNOWS we open these accounts from one device no matter which proxies we used. Now when I log in a brand new account with a brand new private proxy, FB recommends me to join the exact groups I target with the other accounts! Facebook knows we are using proxies, somehow, even when we clear browsing history.
3 years ago I had the same experience with Google. I was trying to activate an Adwords coupon with a brand new account in the USA. So I got a USA proxy but Google kept showing the Goggle front page of my home country no matter what I did.