There is a trick to make this work, you need to publish the layers rendered by the subtype. This writes the domains per field into the renderer and most apps pick this up.
EGDB based feature services include a rest end point, querydataelement, that provides the class info. I believe ArcGIS Pro is the only client reads that.
Hosted feature services do not have this. If they did, clients could be updated to uniformly read this and the renderer would not matter.
I have a post in here that details this, will try to find it.
