Qlik Sense is still a young product. There are lots of things that are not implemented yet. Are that bugs? Or are they missing features. Anyway, at this time it is not possible to assign individual labels to the dimensions in a drill down group. It's also not possible to reuse Master items in drill down groups. Afaik only field names and expressions can be used.
You can hide the formula by setting the Labels and title option to Labels only. Of course that will hide the field name you used as second level in the drill-down group too. I don't know if that's acceptable.