Error (bug?) when creating records from Dynamics CRM updated-mode forms

I'm working on a project with CRM Online, and I just found a significant difference between how records are saved from updated-mode forms as compared to classic-mode forms.

I had added the territory field to the OOTB "account" form, and I also created a new security role based on the OOTB "salesperson" role, but I neglected to add append-to rights to the territory entity. Because of that configuration (actually mis-configuration in this case), I would have expected CRM to throw an error if a user in my custom salesperson role attempted to create an account linked to a territory, but it should have worked fine if the user left territory blank. As it turns out, that only partially worked as expected.

The user could create a new account record with a blank territory from a classic-mode form, but when creating a new record from an updated-mode form, the create process would just sort of hang. Eventually if the user closed the form, a CRM system error message was displayed, and the record never got written to the database. Once I added append-to rights on the territory to my custom "salesperson" role, creating a new record with a blank territory worked fine from both types of forms.

I wonder if there is a reason for this discrepancy or if it's just a bug? If you've had any personal experiences or have any insight, please share in the comments.