How can black castle here? - Maybe Bug report

Okay. This game just happened and I was curious about how I could've screwed my enemy even more. Nonetheless.
We're in this situation here: prnt.sc/vs3h1i (Has to be via a screenshot so the suggested move is visible).
Is this a thing that black is allowed to castle here or is it just a bug?
Everything I did that lead to this:
1. Queue for a game of 5+3
2. My opponent wanted a rematch after he lost his first game
3. After the second game my opponent left the lobby really rapidly (prob not relevant but still listed here for the sake of completeness)
4. I went into the analysis board that is accessible via the post game lobby
5. Up until move 15/16 I went with stockfish only
6. After that I requested a server analysis
7. ~3 more moves and this (bug) occurs

PS: Stockfish not only recommends me the move but the game also allows me to make it. After the castle move it looks like you would expect it to look like after a normal castle. (see this screenshot: prnt.sc/vs3h9z ).

I have not tested it in an actual game but this bug (if it is one) could be gamebreaking if it was actually possible in real games and not only the analysis tool.

Can you link the game?

you can castle

That's called queenside castling (or castling long)
Perfectly legal according to FIDE rules

You can castle because the king is not moving through/ into check. It looks like the rook is interrupting but since you are castling queenside the rook never actually intercepts the king.

it is legal to castle on the queen side in this position

See move 19. The FM was shocked as well^^

yeah guys b8/b1 can be covered when you castle queenside

You wrote quite a lot of words that didn't helped, but you cannot castle only when the *king's* path has been cut off.

www.fide.com/FIDE/handbook/LawsOfChess.pdf

(1) The right to castle has been lost:
[a] if the king has already moved, or [b] with a rook that has already moved.
(2) Castling is prevented temporarily:
[a] if the square on which the king stands, or the square which it must
cross, or the square which it is to occupy, is attacked by one or more of
the opponent's pieces, or [b] if there is any piece between the king and the rook with which castling is
to be effected.

So may castle. Very interesting example this game.