The bash script below generates and umbrela project (mix new --umbrella logcfg) and then an app (mix new demo). It then creates a task and adjusts config for the :default logger. Finally, everything is compiled and the generated task runs. (mix hello). Despite setting :type and :filesync_repeat_interval in the config, the logger still sees the default values:

19:07:52.772 [warning] config.type :standard_io
19:07:52.773 [warning] config.type :no_repeat

Why?
Elixir 1.15.7 (compiled with Erlang/OTP 26)

Koszal

#!/bin/bash

mkdir /tmp/logger-$$
cd    /tmp/logger-$$

mix new --umbrella logcfg
cd logcfg

dsn=`pwd`

cd $dsn/apps
mix new demo

mkdir -p $dsn/apps/demo/lib/mix

cat <<TASK > $dsn/apps/demo/lib/mix/hello.ex
defmodule Mix.Tasks.Hello do
  use Mix.Task
  require Logger

  def run(_) do
    :logger.get_handler_config(:default) |> IO.inspect()

    {:ok, lcfg} = :logger.get_handler_config(:default)
    Logger.warning( "config.type #{ inspect(lcfg[:config][:type]) }" )
    Logger.warning( "config.filesync_repeat_interval #{ inspect(lcfg[:config][:filesync_repeat_interval]) }" )
  end
end
TASK

cat <<CONFIG >> $dsn/config/config.exs
config :logger, :default,
  config: [
    type: :standard_error,
    filesync_repeat_interval: 5000
  ]
CONFIG

cd $dsn
mix compile
mix hello
pwd