When creating/editing a newsletter the Text & HTML blocks are not always showing handles to edit, clone or delete. If they do & attempt to edit, the changes are either not saved or defaults to showing the whole home (front) page inside a block with no handles. All other blocks appear to be working correctly in the composer.
Using Chrome & Firefox, Version 6.9.7 of Newsletter, WordPress 5.5.3 PHP 7.4 Tried disabling all other plugins with same result.
Good news. Turns out CPanel module, ModSecurity, is the culprit. There appears to be some incompatibility with PHP (7.4). If anyone else if having similar issues try disabling ModSecurity and see how you go. Re-enable it when you’re finished.