I can confirm that the problem has been solved as of PowerPoint Version 1908 build 11929.20300. Thanks to Bob and all the people at Microsoft for fixing this!
Sorry, but it did not work for me. Just updated to 1908 build 11929.20300 and the bug is still there. Also, there is no mention in the release notes of this bug being fixed.
However, I desperately need this feature to work and I have been able to restore this feature by reverting to an earlier version of Office 365, to build before the bug was introduced. It is amazingly simple to do. I
followed the steps in this article:
https://www.cnet.com/how-to/how-to-revert-to-the-previous-version-of-office-365/
I am not sure which build the bug was introduced, but I have reverted to Version 1903 build 11425.20244 which I knew worked. The feature is back and I am happy for now.
David