Offline Map: Reference layer not show in ODK Collect

I am using ODK Collect on a Samsung Galaxy Tab A tablet. I have loaded an mbtiles file into the layers folder (i.e. /sdcard/Android/data/ as a reference layer and have selected the basemap as mapbox. When I start my geopoint, geoshape, or geotrace questions, the reference layer is not displaying. Any guesses as to why I have this problem? Does the Mapbox map style need to be set to any particular style for the reference layer to show?

If you are using a recent Collect version I think you are using an old dirrctory.
Collect now use a device directory and can't use th sdcard :

You should find the directory on the phone storage.