The stringer check already reported bearing stress for a bearing length you typed (3.5 in unless changed). It now also turns the question around: how short can the bearing be?
The end reaction V divided by stringer thickness b and the typed allowable compression perpendicular to grain gives the minimum length, V / (b x Fc-perp). No allowable is shipped; you type it from your lumber table.
Hand arithmetic, as in the tests: a 277.8 lb end reaction on a 1.5 in thick stringer with a typed 425 psi needs 277.8 / (1.5 x 425) = 0.436 in, so the default 3.5 in is ample. With a typed 150 psi and 1 in of bearing the stress is 185 psi and the check warns. It checks bearing only: the notch at the support and the connector's own table are not covered, and no real stair has been checked.