What's the reasoning? Is this a side effect of something else or an issue with the field apps (it happens in collector also). Symbology w/o the subtype field works fine in Map Viewer, it's only in the field apps I'm having the issue. It's weird, they'll still come up as related layers to other layers in the map, but any arcade expressions set with them no longer work and the feature don't display on the map (but no loading error). As soon as you switch back to the subtype field the problem is fixed
I can't find anything in the documentation regarding it. It's a pain for me as I have to have subtypes for my field for reasons I won't bore those reading this with.
edit:
Look like it had nothing to do with subtypes but the symbols we were using may not be supported in the field apps for some reason...or possibly some side effect of them; with these symbols removed the issue is gone. Anyone know if there is a reference for symbology and fonts supported by Field Maps and Collector?