Observer in iex -S mix error: (UndefinedFunctionError) function :wx_object.start/3 is undefined

Is there a known issue with :observer with Elixir 1.15 that I’m unaware of? I’ve added :observer to :extra_applications but it gives this error:

iex(1)> :observer.start
** (UndefinedFunctionError) function :wx_object.start/3 is undefined (module :wx_object is not available)
    :wx_object.start(:observer_wx, [], [])
    (observer 2.15.1) observer_wx.erl:70: :observer_wx.start/0
    iex:1: (file)

If I start just iex (as opposed to iex -S mix), it works just fine. Is there something else I must add to the mix config?