Can't remove use of Markdown without editing message body

Environment

• Soapbox version: v3.2.0
• Backend (Mastodon, Pleroma, etc): Rebased
• Browser/OS: Firefox

Bug description

Reproduction steps

1. Create a post with Markdown
2. Edit the post, unchecking Markdown

Expected result:

Post body is unaltered but it goes from using Markdown to not using Markdown

Actual result:

The form goes blank and the Markdown status goes unaltered