Mail deletes the content of my drafts

I save a draft of a message to my Drafts folder. Later, I come back to finish it. I double click on the message in the subject list.

The message comes up in a new window but the body of the text has disappeared and in the message list, the subject of the message has changed to "This message has no content". Furthermore, the new window cannot be closed!

My original text cannot be found in Trash or anywhere else. If I close Mail and re-open it, a duplicate empty message appears and the new window can now be closed.

I have lost several drafts this way but not seen this problem mentioned anywhere.

Using Mojave 10.14.6 on MacBook Pro (13-inch, 2017, Four Thunderbolt 3 Ports), 3.1 GHz Intel Core i5, 8 GB 2133 MHz LPDDR3