I have an editable feature service (not hosted) where the data sits in an enterprise geodatabase (SQL Server) published from Pro 2.4.2 to the hosting server in ArcGIS Enterprise 10.8.1. The service contains several point feature classes and a couple of line feature classes. Not sure if I'm encountering a bug or just something to do with the way I've configured things.
In the Field Maps web app I have made some fairly simple forms for each feature class with a few of the fields set as required and some nice hints and formatting. When it comes to trying to submit a line feature in Field Maps 21.1.1 build 396 on iOS I get the error message below saying Unable to Submit due to an attribute failing - in this case what is shown as the Length field which is Shape__Length in my database.

I am unable to edit the length field on the form in Field Maps and wouldn't want to anyway because it should get automatically calculated for me. I'm also unable to remove the field from the form in Field Maps web app because it is a required field and I don't think I can make it a non-required field (and again probably wouldn't want to).
If I open the same map in Collector (Aurora 20.2.4 build 2851) then the Length field does not appear on the editing pop-up and I can submit features with the field getting automatically populated behind the scenes successfully. If I add the feature service to a new map, don't do any form config in Field Maps web app and then open up in Field Maps iOS client then the Length field is also not shown and I can submit records. In both cases I'm assuming this is because form data is being ignored in Collector and it just isn't there in Field Maps.
I guess I can live with not having any Field Maps form config saved with the map at this stage but would be good to know if this is a known issue that will be resolved in Enterprise 10.9 or if there are any other workarounds?
Thanks in advance
Gareth