How odd! Do you mean you put them in the layers folder in Stockage_interne/Android/data/org.odk.collect.android/files? If so, it may be that your device organizes storage a little differently for whatever reason. What brand is it?
If it's working, I don't really think you need to look further. The /sdcard prefix is quite misleading. These days it generally corresponds to internal storage but still has the name for legacy reasons. Does your device actually have an sd card slot?
We are working on a more direct way to select mbtiles which will be available in the next Collect version.