The formatter is registered for Elixir
not EEx, so I think if it works at all, it would format the Elixir inside the EEx tags.
I’m not sure how formatting a whole file works since the outer (HTML) needs to be formatted, then the template tags (EEx) and then the inner (Elixir). IntelliJ Ultimate Edition, WebStorm, PHPStorm, and Rubymine have HTML formatters I think, so they should format the HTML, but I don’t think HTML support is in IntelliJ Community Edition.
So, maybe I need to register a formatter for EEx too, but I’m not sure how that would work since you can put the tags wherever, like to interpolate an attribute or even seemly invalid, partial HTML and that formatting would be dependent on the outer language.
Which IDEs have you tried Auto-Indent Lines in?