Thank you for your post. This is easily reproducible on both Macintosh and Windows computers.
I have forwarded your information (as well as my findings) to our Development and Software Quality Assurance (Testing) departments for review and confirmation. When I receive more information, I will let you know.
As an additional note, if the portal is not present on the layout, you can edit the record in Instant Web Publishing.
I received information back from Development.
This happens because fields on the main and related table point to the same fields on the same layout. For the example provided, Instant Web Publishing allows to edit fields via the portal (and the changes are accepted). If a user wants to change data via the main table, he/she needs to place portal control on the layout first, and then add the fields from the main table. That will allow to change fields via the main table. Chapter 4 of the Instant Web Publishing Guide, "Session: Tips for rendering layouts in Instant Web Publishing" states, "Avoid using more than one copy of the same field on a layout. Otherwise, web users must change all copies of the field to save changes or to perform a find."
I have duplicated this result by adding the portal first before the main field. This time, the changes to the portal are ignored, but the changes to the field are saved.
If you need clarification, please let me know.