This might better belong i the Web App builder section, but I found a similar problem and solution here. @AlixVezina, I am hoping you could take a look at this.
I've configured a map that has a relational feature service in it. I built a Web App builder application on the map. I have configured the settings to allow for editing. The Create new feature icon is missing from the related features entries at the bottom of the Smart Editor Widget control panel:

I note that in the Configure Smart Editor dialogue, in the Custom settings for the parent layer, that Update only is enforced for the child layers. I have looked through
the layer and the map settings, and do not understand what is enforcing this. In the layer settings, for instance, geometry updates are enabled for all layers in the service.
Any thought on how to release the lock on Updates Only?
