Before images in Develop module are blank in Lightrrom Classic 15.4.1

After upgrading to Lightroom Classic 15.4.1, I find that the Before image in the Develop module is often blank.  I have undertaken all of the possible troubleshooting steps with no success.

Downgrading to 15.3.1 solves the problem in a new catalog but if I then upgrade that new catalog to 15.4.1 the problem returns.

Is this a known bug???