Be default Texts saves Markdown with Setex headings (underlined) at the top two levels. Optionally it can use ATX style headings (#
characters) on all levels.
Execute the followting command in Terminal app.
defaults write io.texts.Texts TSAtxHeadings -bool yes
Download and install atx-headings.reg to use ATX style headings for all levels or setex-headings.reg to switch back to Setex headings.
Please contact support@texts.io if you have any questions.