>> I'm trying various completions after customizing completions-sort to nil,
>> so currently noticed a problem in the completions of file names.
>
> So you agree that the problem is wider than that?

The problem reported by this bug report is that the order is random by
`C-h v M-n M-n' because it uses obarray.  There is no such problem for
`C-x C-f M-n M-n' because the list of default values is truncated
explicitly in `read-file-name-default' by

  (minibuffer-with-setup-hook
      (lambda ()
        (setq-local minibuffer-default-add-function
          ...

So maybe to use the same to truncate the list of default values
for `C-h f', `C-h v', `C-h o'?