I've successfully used Arcade, in both Map Viewer and Map Viewer Classic, to develop pop-ups that pull data from a related table, using an expression similar to the one below:
var tableRel = OrderBy(FeatureSetByRelationshipName($feature,"",["*"],false),'assessment_time DESC');
This pop-up expression can be saved w/o error and, along with other lines of arcade code, returns data from the related table. However, when I try to use that same expression (in a same map) in a Symbology expression, I receive an error and am unable to save the expression at all. Has anyone found a way around this strange restriction?