I know you say latest Edge but are you using Classic Edge or Chromium Edge? I think this can only be headless in chromium edge (but you could certainly try the headless option below).
You would be able to tell based on if you had sent in the chromium option:
Open Browser | url | edge | options=use_chromium=True |
To make it headless it would be:
Open Browser | url | edge | options=use_chromium=True;headless=True |
You will also need to be using selenium 4 or the plugin in the link below.
More info here: Selenium Library with Chromium Edge Browser