I've found that the fix mentioned above by gazumped (hit enter in the note, Ctrl-Z to undo, click on another note, and then back to the original and editing then works) seems to serve as a permanent solution for an individual table, so the problem isn't as bad as I initially feared - though of course having to do that even once for each table is a bit of an irritation. All the tables I've experienced the problem on are now back to normal - though I'm sure I'll keep encountering the issue for a long time as I've got masses of tables I don't refer to very often!
Hope it gets fixed soon! In the meantime I've been managing to make some minor changes to the content of tables by highlighting the text I want to edit or remove, then pressing the space bar or retyping the new content - ie trying to avoid using the delete or enter buttons (though I keep hitting them without thinking!) I do like the new-style tables though. In particular, I have a template with half a dozen columns that I use for my monthly calendar. With the old version, every month's calendar ends up with completely different column widths - so this will be a lot neater!