We use an offline mbtile to show the ground and the grid of cells to visit in the form.
Even if map settings are ok and the mbtiles shown as selected in the "layer menu" of the map, mbtiles does not appear. We need to uncheck/check it each time to show it on the map.
This does not occurs with MapBox renderer but this one can't show too much select options on the map for the moment.
There's a lot going on with geo so for anyone making heavy use of maps, please do be on the lookout for things that don't seem quite right. We do our best to verify most feature combinations but there are a lot (e.g. osm engine + offline maps + multiple questions in a row). Thanks for doing the comparison with Mapbox for this one!