A stringer's end reaction has to go somewhere, and wood crushes across the grain long before it breaks along it. stringerCheck already reported the reaction and how many connectors carry it. It now also divides that reaction by the stringer's thickness times a bearing length (3.5 in unless you type bearingLength) and compares the result with a bearingFcPerp you type from your lumber table. Nothing is shipped as a default.
Hand arithmetic, as in the tests: 50 psf on a 16 in stringer spacing over a 100 in run gives a 277.8 lb reaction. On 1.5 x 3.5 in of bearing that is 52.9 psi. If the stringer only bears 1 in, it is 185.2 psi, and a typed 150 psi allowable fails with a note to lengthen the bearing or add a ledger.
Not covered: the notch at the support, ledger versus slab, and the connector maker's own table. No real stair has been run through it.