I know this is a conflict somewhere between the Leaflet maps, the Map extension, MediaWiki, and the MonoBook skin involving CSS, but I don't know how to figure it where exactly the problem lies.
I have a new MediaWiki install (version 1.38.1, default skin = monobook) with the Maps extension (version 9.0.7), and maps I've added to pages work as described on the documentation pages.
The MonoBook skin with responsive mode checked mostly behaves as expected. But when a page gets narrow (as on mobile), about 550px or less, images and maps go from their normal left-aligned behavior to full width. Images become centered (OK), and maps become full width (OK), but the OSM map tiles disappear.
Part of what makes this more confusing to me is that the polygon for the map doesn't disappear, just the map tiles.
You can see this behavior here:
Either resize your browser window narrower until the tiles disappear, or bring up the developer tools in Chrome, et al, toggle the device toolbar, then resize the simulator window until the tiles disappear.