Can't Invite a Certain Player to 3s Flex

I'm ranked plat in 3's flex, so I can invite all of my friends who are gold, plat, or diamond in flex, except for one guy who's plat 1, Donahue, it says he's an ineligible duo partner. The workaround is to manually type in his name, even though he's grayed out on that list. I used to be able to invite him okay though. I'm on the old client.