I reported this issue to Dusan in December but still waiting on it to be fixed so hoping for a workaround as MapWel is utterly useless to me otherwise.
Both with OSM data imported within MapWel and from extracts exported from the OSM website or through JOSM:
MapWel cannot handle objects with long IDs. I don't know if it's like an integer limit on MapWel (this is both the 32-bit and 64-bit version of MapWel Advanced Routable) but any entity with a "long" ID is simply ignored.
If I edit the IDs of objects to make them shorter, MapWel can see and render them.
Maybe some application that can rewrite all the IDs in an OSM or any XML file so turn 394872346, 197459871 etc to 1, 2 etc?