Hi,
I have been using GeoServer 2.20.3 and latest ArcGIS runtime SDK for .Net 100.13.1, and the WMTS layer with EPSG4326 are not visible in the Scene.
The Scene spatial reference was set by default EPSG4326, but the debug message from LayerViewStateChanged event show error "Spatial reference invalid or incompatible: The Spatial Reference is invalid or incompatible with the Map's".