I'm not real familiar with the Harmony remotes, but I know folks sometimes have trouble untangling the specific commands tied to an activity. The workaround of switching to a different activity and back to the OPPO would fit with that - it suggests to me that the Harmony has a command to change from "DVD" to "7.1D" under the OPPO activity but not any way to get back without re-starting the activity from scratch.
If you can manually add a command to the OPPO's activity, you could try learning the "DVD" input command to it and use that as a way to directly switch back. (Assuming, of course, that the "DVD" input is the one you are using with the 980H's digital audio connection...)