A small and obscure tweak in Pro did the trick! Thank you for posting this.
I am publishing from an Exterprise Geodatabase on a stand-alone ArcGIS Server (10.9.1) using Pro 3.03. In field Maps Designer, I could not set my default values as they were not showing up. By toggling these in the manage templates panel in Pro and re-publishing, the defaults showed up in Field Maps Designer and are working in the Field Maps app. Thank you!