This seems to have the same issue I saw with Google Maps where it directed me along 29th St from near Mission Bernal Safeway to Diamond Heights Safeway. This includes a 22.7% grade! It's much faster and easier (at least on my cargo e-bike) to take the longer way around up Clipper St (12% grade).

The bug seems to be apparent when you set the end point at Duncan & Diamond Heights Blvd and the start point directly north along Clipper.

This route seems to maybe save 1 ft of climbing but goes up a 23% grade.

https://flattensf.com/#t~-122.43968~37.74865~-122.44028~37.7...

Half a block up the street it takes the sensible route with only a 13% grade. (Between the two different starting points is 41ft of climbing.)

https://flattensf.com/#t~-122.44087~37.74864~-122.44028~37.7...