How exactly do you create that new grandchild record? Is "allow creation" enabled for the portal in that relationship? I would assume so from your description.
Is there a filter on this portal?
Don't have time to watch the video now, but this sounds like the standard "Need layout solution for nested portals..." or "master slave" pair of portals. Some detail appears to be different though or I would not expect the newly created record to disappear like this.
Don't worry I worked out what was going on. I need to click outside the Portals, on to the main record to make these changes appear. I also made a script using the Go to Related Record command to reselect the main record and added it as a script trigger on committing the changes. That way it didn't matter where I clicked it would appear immediately.
Clicking a blank area of the layout commits the record. So you should be able to add a commit record step to a trigger controlled script to automate this.