Gutenberg Preview Problem

I have been getting a problem with the Gutenberg page/post preview not working until after I click publish.

Using the “Health Check & Troubleshooting” plugin I switched to a default theme and turned off all plugins.

I went to edit an existing page and deleted all the blocks (there was no CSS) – preview was working and the page was blank. So all good.

Then repeated with “WP Add Custom CSS” plugin enabled – the preview was of the page before deletion – so wrong.

I repeated this a number of times and got the same result.

Can someone please confirm this result so we can get it fixed – thanks.