what is your expression and where are you putting it
The Arcade expression sums up the records from related table and it's used in pop-up to a Web map.
When I try to open the map into Field Maps for updating the map the pop-up refuses to display normally. In fact the pop-up doen't come up at all (not only the Arcvade expression but any field in pop-up.
There are several posts that relationship by name does not work right. See here https://community.esri.com/t5/arcgis-field-maps-questions/add-inspection-related-record-bring-value-...
Workaround is to use Feature set by name and do the filter with a query.
Hope that helps
I would suggest configuring the pop up in Pro and publishing it. the new service should show it the desired way.