WMTS with valid "time" dimension (according to OGC standards) can be displayed in ArcGIS Portal in e.g. a webmap (as layer or basemap). When using the same webmap as base of ArcGIS field maps mobile app, the WMTS is not shown (see attachment). I tested the WMTS without time parameter in the app - then it worked.
The same issue occurs when using the webmap in ArcGIS Survey123.