Layers published to the portal with Arcade configured pop-ups, when saved as a web map and opened in ArcGIS Pro, break the pop-up expressions and return the error message "Invalid expression"

Workaround

Use the Helpers section to use the feature set by relationship name.

Click the functions Filter > select 'Feature Set by Relationship Class'.  

Expression:

var x = FeatureSetByRelationshipName($feature,"<double click relationshipclass name from the filters>"

var y = count (x)

console (y)

return y

The expression validates.

Steps to Reproduce