Support for group layer in the Runtime API will be in 100.5, due to be released in early April. As a developer you will be able create and use group layers with operational layers in a map or scene. Persistence is a different matter. The scene spec (mobile and web) has a formalized definition for group layer. Runtime 100.5 will honor group layers in a web scene or mobile scene package. The map spec (mobile and web) does not have a formalized definition for group layer. When this is formalized, Runtime will be able to read/write maps with group layers. ArcGIS Pro 2.3 just happens to write group layers into the mobile map. Runtime 100.5 does not read group layers in a mobile map. We are working to formalize the definition of group layers in the map spec (and include properties such as min and max scale) for ArcGIS Pro 2.4 and Runtime 100.6.
Hope this helps,