Using a pointer from Greg Peirce I learned that, in James Thomson’s words, “in Tahoe the Mac App Store is completely broken if you have the TestFlight version of the app installed when you try to download the release version from the MAS.”
Adapting his solution, I was able to reinstall OmniFocus 4, by installing a Homebrew app “mas” and then using the id number (which can be found for any app by looking at the app in your list of Purchased apps in the MAS, and then selection the three dots and choosing “copy” – see the screenshot below).
In Terminal, type (without quotes): brew install mas
Then: mas install 1542143627
