How to customize message in maestro mail notification
I know this has been up for a while but thought I would answer. Up until recently, Maestro was removing any HTML from that textbox using Drupal's checkPlain.

I added a patch which was accepted into the development branch so it should be sorted in the next release of Maestro. If you wanted a fix in the meantime, use the patch file here: https://www.drupal.org/project/maestro/issues/3076536