There isn’t any other solution than embedding only Facebook content that isn’t needing login.
But I reckon that was some glitch from Facebook itself, because the situation is different nowadays.
I’m oneboxing now from same group than I did at OP, using DiscourseHub like earlier and I’m quite sure CDCK doesn’t change location of theirs server. And I get onebox in finnish as expected because I’m a finn, but unexpected because we are using english here:
So I would say we are in situation where we can’t do anything. Facebook gives that kind meta they want and that’s it.