rhm

It sounds to me like your saddle is too wide. Possibly the edge is too hard; but if the saddle were narrower there, that wouldn't matter.

I am very doubtful about the utility of measuring your "sit bones," which are shaped kinda like the rockers on a rocking chair, but backwards; on a person they are close together in front and farther apart in back The points at which they contact the saddle are closer together if you tilt your pelvis forward, and farther apart if you sit relatively upright. To measure them correctly you have to mimic your position on the bike-- and this is the detail I never see in the "how to measure your sit bones" instructions.

That said, on the MTB you probably had a relatively upright position, so perhaps my misgivings are unwarranted in this case.