"Scene filters. UC Surface mix scenes store the settings of the entire mixer. You can choose to load the entire scene or filter various portions of it." (StudioLiveSoftware_ReferenceManual-EN-V7_29092017, section 5.5.3)

I had been testing this with (empty) DCA groups and saw that it works. I would need to check it with subgroups, but I guess ANY assign setting is recallable. I suppose your problem has to do with input channels assigned to the subgroups, which need to configured exactly as the scene/setting you are adding the subgroups to. Don't know if it's worth the effort. Create your scene, create subgroups and assign the channels you want to be part of it, store the scene - it is not really much work and an easy workflow...

May I ask what do you want to do with the subgroups? Why do you want to add them to an existing scene? Wouldn't it be better to use DCA groups?