It’s happend in both leagues run now . Two new owners waiting for approval, I approve the request and boom! They’re gone .
Can’t even search the username to tell them .
Also I can’t edit league to auto-accept them

It happened to me trying to get Hellbringer approved for Michigan Panthers in USFL. I hit approve but it didn't let him join.

JDB got it fixed in USFL. If it's still happening in your league, contact JDB to fix it. It didn't take him very long.