using :required_message (as mentioned in the doc), first did nothing, but it was probably because the dev server was lagging or something like that because after a restart of the server
mix phx.server
it worked
in my experience,
the dev server sometimes (not frequently) lags or seems wrong after many edits in the files or duplication of files then change of the module name.
But everything gets right after a restart or in rare cases after deletion of _build and recompilation from zero