I tried a different brokerage and downloaded one 1099DIV form as a test. But with that new brokerage and just one form, when I tried to "update" it at the Investment Income section, I got again a summary page (from all accounts) that looked like what I attached above. There's no apparent way to drill back down into each brokerage account after downloading. (I guess there's a fallback typing directly into the final forms, but I would rather not do that.)
There appears to be no way to edit data in the GUI after it has been downloaded from the brokerage. Something new in my 15+ years using TT. It's hard to believe others aren't seeing this and I'm trying to figure out what I'm doing differently.
