My settings:
I have checked the "reliefmap"-checkbox in the "reliefmap"-menu.
And in the "mapsforge-settings"-Menu I've checked the second-last checkbox. I'm a little confused by the fact that in this menu there ate two checkboxes (last and second-last, only one of them can be checked) which say nearly the same (in the german translation). What's the difference between those two settings?
I'm using a Nexus 5X running Android 8.1.0
Workaround:
When the shading isn't shown and I open my file-browser (Solid Explorer), open the folder where the .hgt-files are stored (internal memory > oruxmaps > dem), and return to OruxMaps the map is reloaded and the hill-shading is shown regardless the zoom-step. But when I close the SolidExplorer app, the hill-shading won't be shown anymore in OruxMaps.