Thanks for the follow-up, Martin. I had already set the mode to Markdown on Sunday. I had then logged in with a test account on Monday, in a separate browser profile, which received the Rich editor instead.
Also, if that helps, each time I set “default_composition_mode” to “Markdown”, I get the warning: “Would you like to apply this change historically? This will change preferences for 61 existing users.” I understand I should get this the first time, but it seems like a bug that it prompts me each time for some number of users.
For further testing, I also just logged into another test account that hadn’t logged in for 10 months. It was also set to Rich Text mode by default.