I don't see how you can defend both pawns at once if the bishop is located on the f8 square. Where did Stockfish go wrong in this game?
[FEN "5B2/3k4/2p4p/4n1r1/8/2R5/6K1/8 w - - 0 1"]
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I am not 100% sure of what your are asking.
Are you saying that Stockfish missed a draw by playing Bf8 as shown in the diagram ie there was some other move that could have drawn?
Or are you saying Stockfish missed a drawing line later.
Or are you asking in general where Stockfish went wrong in the game?
If you look at the evaluations then Stockfish seemed to assess itself as losing many moves earlier. Leela on the other hand thought her position better for quite some time but not necessarily a forced win.
In the lead up to your diagrammed position Stockfish I think is lost and has been for quite a long while. The Black pawns are wide apart making the defensive Bishop's task very hard. The White K has to keep an eye on one of the pawns whilst the White Rook keeps an eye on the other pawn whilst at the same time trying to harrass and keep out the Black K with checks.
Black has to manoevre carefully but will always be able to shepherd the c Pawn down to the queening square.
I think Stockfish made a strategic error with 40. Ng2 allowing Leela to swap her bad Bishop and get a position with the "Petrosian Knight" verses Stockfish's bad Bishop. After this swap Leela's Q invaded, Stockfish's position deteriorated leaving a difficult defensive task.
Apologies if I have missed your point.