Using this map:
Hike&Bike http://a.tiles.wmflabs.org/hikebike/$z/$x/$y.png

the map does not display until one moves the slippy map around then the off-screen part is seen as "painted" and you can then do enough movements to get the map displayed.

OpenTopo https://a.tile.opentopomap.org/$z/$x/$y.png

does not display at all, even though the URL works in browser. See for eg. https://a.tile.opentopomap.org/14/8695/5583.png

Looks good.