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I think I see what happened.  The FEN for the starting position is this:
1nbackqbn1/pppppppppp/10/10/10/10/PPPPPPPPPP/RNBACK1BNR w KQkq - 0 1

Black doesn't have any rooks, but according to the FEN, both players
still have castling rights (KQkq).  I think that makes it an illegal FEN,
and it probably should have been this:
1nbackqbn1/pppppppppp/10/10/10/10/PPPPPPPPPP/RNBACK1BNR w KQ - 0 1

I see 3 ways an engine could handle this:
a) It could reject the FEN.
b) It could accept the FEN but not grant any illegal castling rights (if
the rook is missing or the king is in the wrong place).
c) It could accept the FEN and always grant the castling rights.

Sounds like Joker80 does c), although I'd tend to prefer b).