Our field crews are reporting that they are unable to edit the time portion of a date field in Field Maps. I am able to replicate this on my device, but I can edit the same field on the online map viewer. The layer in question is a Feature Service referencing data in an Enterprise Geodatabase. I'm using Portal 10.8.1 and Field Maps 21.3.1 (Android). I do not currently have the Field Maps app installed to the Portal. Is there a configuration that I'm missing somwhere, or is this the standard behavior?
In your case, since you are using the pop-up for editing (and not the newer forms that use the Field Maps web app), check the pop-up configuration for the date field. If the format does not include time, then the time picker will not be displayed.
I did have the time format enabled, but it wasn't showing. I ended up installing the Field Maps web app and tinkering with the form. It's essentially showing what I had before, but time editing is available now.