I'll admit, after installing Field Maps web app and map viewer beta (latest ver4), I'm surprised you can't apply Arcade expressions directly to attribute fields to calculate values. The required checkbox is awesome on the form creator in FM web app.
I noticed in map viewer beta, when you create/specify an Arcade expression for the popup title, it displays "Untitled Feature" when you create a new feature. After the feature is created, when you click it to open the popup, the Arcade expression fires like it should. This is not the case with regular map viewer.
Here's the doc that mentions expressions in popups. Calculate the title or entries using Arcade expressions
Field Maps Android mobile app works as expected when you create a new feature; the Arcade expr. is displayed correctly in the popup title (this popup is actually the new feature create template).
Field Maps iOS mobile app does not work; the Arcade expression in popup title of new feature creation is blank. This behaves like map viewer beta.
Arcade is inconsistent not just across platforms but within platform. I'm having issues with Arcade in my popup titles within Webapp builder. If I am viewing the app within desktop scale the title shows, however if I view it from a mobile scale the arcade title will not show. This is regardless of the size of the string. I can have normal text in the title which have more characters than the arcade one and they will show.