What would cause the Field Maps app to not accept the arcade expressions, although they work in the map viewer in the Portal for ArcGIS and ArcGIS Collector?
Field maps app 22.4.2 on two IOS devices 15.3.1
These arcade expressions are coming from ArcGIS Pro when publishing the layers to Portal
Any chance you should share the expressions in question? There are a few differences between Pro and the web when it comes to Arcade, so it's possible your expression wouldn't cleanly transfer between the two. Hard to say without seeing the expression, though.