>> It's literally just a global minor mode defined for your test minor
>> mode, basically `(define-global-minor-mode test-global-mode test-mode
>> (lambda() t))`
>
> In that case, you should disable (or rather, not enable) that minor mode
> for minibuffers, for example:
>
> (define-global-minor-mode test-global-mode test-mode (lambda () (or
> (minibufferp) (test-mode 1))))
>
(Note that this does not answer your question "In Emacs 27, turning the
minor mode on inside the minibuffer would not override the key map given
to read-from-minibuffer", with the above the effect is the same in Emacs
27 and 28 AFAICS. Without seeing the minor mode, I cannot help you more,
I don't have a crystal ball.)