Thanks, I had automatic map loading enabled for both raster and vector maps, but what I hadn't enabled was pre-loading of the global map. Once I turned that on, the auto switching between the global map and the vector OpenAndroMaps started working.
However, there are still problems switching between vector maps. Here's what I see when from my home location in Australia I search for Valence, which is on the France_South map, so I'm switching from one OpenAndroMap to another. I have to do a zoom in or out to get the entire screen showing the map:

As well as OpenAndroMaps I also have a couple of LoMaps installed (for Christmas Island and Singapore, which avoids having to have 2.5Gb of the OpenAndroMap Malaysia-Indonesia map installed just to get those two small areas), and there are problems switching between OpenAndroMap and LoMap vector maps. This is what I see if I search for a location on Christmas Island, involving a switch from an OpenAndroMap to an LoMap. To get the map to display here, I have to open the map menu and tap on the map for the location:

And this is what happens going in the other direction - having an LoMap displaying and telling Locus to centre the map at my current location, which is on an OpenAndroMap. The POIs show up, but not the map itself. I have to similarly open the map menu and tap on the map for the area to get it to display:
