Thunderbird defaults to plain text unless you specify a style change, such as bold or italic, or embed a photo. That is one of its key strenghts, not available from other email clients. If you want HTML, regardless of whether it is needed, there is a setting in settings>composition to specify HTML all the time.

Hi - I have that configuration set, and the problem has only started recently

You might try troubleshooting with all addons disabled. The 'good news' is that sending plain text where there were no intended style changes still reach the recipient, allowing the recipient to display text in their preferred font and size, not that of the sender.