Experience Builder doesn't give you the same access to Arcade that you have in other apps. It will honor Arcade expressions in a Details widget, but not when piping attributes into a Text widget. Even looking at the config JSON of something like that, it's not clear how it's deciding the date format. Doesn't seem to be an exposed user setting, but it would be great if it were! You should suggest it as an idea.
- Josh Carlson
Kendall County GIS