Thanks for your response. I followed your procedure to create a test page, but I still can't add anything to the editable region no matter which view I'm in. Have to resort to DW CS6 to add content which then makes it editable in DW 2019.
Edit: Yesterday I noticed that if I applied source formatting the colour of the editable tags changed to grey and I could add code as expected, but when I tried this morning it wouldn't happen, but tried later and it's working again. Something decidedly odd going on!
Edit II: I've realised that for the 'applied source formatting' trick to work the page needs to have been template updated. So for anyone experiencing the same issue:
1st time: Apply Source Formatting
2nd time: Tools -> Templates -> Update Current Page -> Apply Source Formatting
Seems likely it's a bug which I'll report to Adobe.