For anyone still dealing with this issue. I've found the cause and workaround when publishing from Pro to ArcGIS Server.
If the feature template inherits the default values from the feature class schema, even though it appears in the Pro feature template, it will not be pushed into the published service and therefore will not work in Field Maps. As others have mentioned, for it to publish with the service, you need to trick it into thinking the default is uniquely set for that feature template. However, you don't need to do as many steps as others have suggested (change, save, change back, save, republish). You can simply change each default in the feature template properties to anything else, and then change it back to the default before closing the properties. You can do this to all of your feature templates before saving the project and re-publishing the service.
One trick to determine if a default value is going to publish, is to look for a Grey bar next to that field name while in the feature template properties. If there is a Grey bar next to the field, then it's recognized as a unique default and will publish. When tricking it, you want it to appear with a Green bar next to the field, meaning it thinks it has changed. After accepting the properties the Green bar will change to Grey. If the bar next to the field is Blue, then it will not publish. If you to click on a default drop down and select the same value, you will see the field remains Blue because it knows it is the same value and nothing changed. This is why you must first change it to something else and then back to trick it (no need to close in between).
In the example below, 'Enabled' and 'AncRole' have Green bars next to them indicating they have been altered since opening the properties (inherited default was changed to something else and then back to the default). After closing and reopening the properties, they will have a Grey bar next to them, as is seen next to 'Segment' and 'GIS Status' which were previously changed. 'Owner' has a Blue bar next to it even though it has a default value, as do all the field without default values. In the current state, 'Owner' would not have the default published with the service, but the other 4 fields will.
