I'll be as brief as I can.

I'm working on a fleet of 34 bicycles for a high school. They are 2023-24(ish) KHS Winslow mountain bikes. They do not know the year, but the bikes are 1-1.5 years old.
RDs are S-Ride, RD-M300C, 9-Speed
Shifters are S-Ride, SL-M300C, 9-Speed
Cassettes are CS-M300C, 11-42T, 9-Speed

https://khsbicycles.com/bikes/2021-k.../winslow-2021/

Of the 34, 16 are trashed; broken hangers, busted RDs, sprockets bent in the cassettes, crimped brake hoses, chain plates are plied apart, etc. Parts are strewn everywhere.

I went into my LBS for parts, I even brought in one of the bikes and the spec sheet, link above. The LBS does not have the spec'd S-Ride RDs or cassettes, they do not even have 11-42T cassette, so I had to get 11-36T (the kids won't notice). LBS sold me:
Altus RD-M370-SGS rear derailleur
CS-HG400-9 rear cassette 11-36T

First, I put on a brand new hanger from derailleurhanger.com, hanger number 206 and it fits perfectly. I even checked it to be sure it was straight using my hanger alignment tool. I only mention it in case someone asks.

Second, this is the main issue, when I mount the new 11-36T cassette and the new Altus RD, using the stock S-Ride shifter , I am unable to adjust the indexing, it will ALWAYS shift up or down the cassette 2 sprockets at a time.

I pulled off an S-Ride RD and cassette from another bike (I was losing my mind at this point), mounted those on THIS bike, the shifting is fine.

I guess it must be the lever throw on the shifter? I would have thought that the distance between sprockets on the S-Ride cassette would be the same on the Shimano cassette.

Thank you in advance.