I have Portal 10.6.1 and I'm seeing the same behavior. I've published several feature layers for testing and can't get the Arcade editor to appear. When I click Calculate Field I am taken straight into the SQL editor and am not given an option to choose Arcade.
I read this and I've changed the editing settings on my layers based on the note under #4 but I can never use Arcade to calculate field values. Using Arcade for symbology via the Map Viewer works fine.
Liana Smith did you contact Esri support? What did they say?