Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>>>> Or if eventually a defface or a defcustom makes its way into
>>>>> display-fill-column-indicator.el, of course.
>>>
>>> Actually, there is a defcustom there (in the expansion of
>>> `define-globalized-minor-mode`).
>>
>> Sorry, I wasn't clear enough.  There is no defcustom or defface whose
>> :group is display-fill-column-indicator.
>
> `global-display-fill-column-indicator-mode` should definitely belong to
> the `display-fill-column-indicator` group (because the `defcustom`
> doesn't have an explicit `:group` so it should fallback to using the
> last-defined group).

Is it, though?
emacs -Q
M-x customize-option RET global-display-fill-column-indicator-mode
And near the end of buffer I read:
Groups: Global Display Fill Column Indicator

Furthermore, the list
(global-display-fill-column-indicator-mode custom-variable) is not a
member of the custom-group property of display-fill-column-indicator.

> BTW, another quick-fix might be to move the defface definition to
> display-fill-column-indicator.el.

Right.