Got a support ticket open as well and it has been escalated up a few times. Been happening to us for about 3-4 weeks now and our only solution is to refresh the email while we are editing it to be able to edit the text version after editing the HTML version. Did get on the phone with support and show them the issue and they were able to recreate it but still no resolution.
I've had a ticket open for two weeks on this - yesterday it was updated that they were just starting to hear reports that others were having this issue, too. Keep opening the tickets!
Agreed! This is not a new bug, it's been floating around since the new email editor was put in. The more tickets we get, the better chance we have of it getting fixed.