Indeed, Apple's 1st unlock of my 6S+ only lasted 24 hours, it re-locked again to some random iCloud email. Glad I held it before returning to Gazelle for the second time. I contacted Apple again and my case was escalated to "a higher level" and whatever that team seemed to do held long enough for Gazelle to process my sale to them.Some people have reported repeat issues.
My advice ... keep a record of this, hopefully you have a case number that you can refer to if you ever need this addresses again someday.