Hi everyone,
to wrap this one up:
With support from... support, I rechecked all settings, ni that process I put the map source from offline vector to online back to offline vector. That seemed to fix it.
Some additional info from support:
In case you can enable the plugin, but do not see the contour lines, please make sure that you do not use any underlay/overlay maps and have Offline Vector maps set as the Map Source in Configure map menu. Also, please note that you need to download offline contour line maps along with the standard maps of the regions you want to view in Download maps.
Please make sure that you use the same zoom level to view the map, which is selected for the Contour Lines. You can change the zoom level for the Contour lines display in OsmAnd menu - Configure map - Contour Lines.
and
This may have happened because of the cache files. If this issue ever reoccurs, please try clearing the cache for OsmAnd app and restart the device.