That's an unusual place for a crack.
Any chrome? I have seen seatstays that rusted out from the inside due to some chrome-plating acid being retained inside. The crack is right about where any little puddle of liquid inside would tend to stay while the bike is 'rubber side down'.

Are there vent holes? Preferably two, near each end, for best flushing of the interior. Even brazing flux is a little corrosive and should be thoroughly washed out.

On two of the frames I saw with the seatstay rotted out from chroming acid, the FB had tried to prevent acid getting in by filling the vent holes with brass braze, but that's difficult to do without leaving a little pinhole or porosity that lets a little acid in, but prevents proper rinsing afterward.

I'm not saying that's a likely cause, just one possibility.

Mark B