In creating a field map for asset collections, all layers appear within the Collect drop down options and are fully functional within Field Maps except for the Hydraulic Elevations layer that has a relationship class to the feature classes for each asset type. The layer is functional within the Webmap when accessed through Portal, but the only way to create an object in the Elevations layer within Field Maps is by selecting a related feature and adding it through the related data. If there weren't instances that required an Elevation to be collected with no association to an asset this would be tolerable, but that is not the case.
There were initial problems with the Elevations layer not showing up within the Webmap at all, but the problem has slightly changed to the current issue after specifying the default Z value for new records.