Well, the problem seems to have been solved by updating the LItchi app version from 2.6.7 to the latest 2.7.1 .
I had a whole series of trials set up today to explore different combinations of the photo interval settings. When I launched and started my trials, I found everything worked, both photo and video. No matter what I had set up on the Interval feature, it worked.
My post #19 from last night reported that I'd been running Litchi 2.6.7 when I encountered the problem. Sometime later last night I decided to go ahead with the version update to 2.7.1. on the iPad app. It's noteworthy that 2.6.7 version for the iOS app was BEFORE the addition of the new photo interval feature. It appears that the missions I set up on the HUB were assuming the 2.7.1 app, but I was still running the 2.6.7 iOS app that did not include that feature. At least that's my mental picture, and I'll stick with it as long as it works. I'm normally one of those guys that doesn't update firmware or apps so long as the older versions work. I think this may be an example of a reason to keep up if you want to design with the HUB and implement with the app.
If all this is correct, I'll have good performance tomorrow on an away construction site that gave me trouble last month on a Litchi mission.