Ack! In the beta just before, Mapbox drew the polygons under the selected offline layer as well. Changing layers didn't make them visible. Only switching to no layer would show them.
After testing all other map engines, and going back to Mapbox, (Google has the same issue as previously described. OSM/USGS/Carto are ok also) normal behaviour was restored and I couldn't disappear them again.
I realised I had added the tilesets and then couldn't see my polygons, so to repeat the behaviour I followed these steps;
- I went to a different location, saw the polygons with no tileset in this area - ok
- used the 'add offline layers' function - ok
- selected the layer - ok
- the polygons disappeared - not ok! (they're still selectable if you know where to tap)
- changed the layer to 'none' - ok polygon appears
- change the layer back - not ok! polygon disappears
- exit map widget and reopen - ok! polygon appears.
Edit: I haven't tried these particular steps in non-beta, and haven't tried adding new tilesets in other engines in beta or non-beta