I am currently experiencing what I think may be a bug with the current version of the "Edit" widget in AGOL's Web App Builder (WAB). I have two hosted feature layers, one line feature layer and one point feature layer that have a relationship class that links them using a Global ID and GUID. Basically I have created an application that is supposed to track road improvement information, where the roads are the lines and the points are the improvements. When trying to add a road improvement point using the "Edit" widget in WAB there seems to be an issue with the new point establishing a relationship to the selected line feature. In some instances I have no issue and the point will correctly associate/relate itself the correct line feature and other times the point will be created, but no relationship will be established. There does not seem to be any consistency or pattern for when the editing process works and when it does not work.
I have also tried this same process using collector on a mobile device and I do not have the same issue; the point will be created and establish a relationship with the correct line feature every time.
For editing relationship records, you may need to use "Smart Editor widget". Here is the web help link for your information:
I've run into this issue as well. The problem is intermittent when using the Edit Widget but the relationships are built properly when I use Collector. Using the Smart Editor widget doesn't allow you to add a new (related child) feature - you can edit existing features that are already related to the parent feature.