Hi Norbert,
did you by any chance find a solution for this? I seem to be struggling with the exact same behavior. Saving text in Markdown format works everywhere (incl. in Wiki description) except in the body of Wiki pages. It always reverts back to HTML. I have experimented with Marklar Editor (https://moodle.org/plugins/editor_marklar) but it doesn't seem to remedy the issue.
Best regards, Ralitsa