A child frame is supposed to show a minibuffer unless it is created with
(minibuffer . nil) or some value specifying a minibuffer on some other
frame. How precisely is your child frame created?
Sorry, my terminology may not be correct, I meant there are two "input areas" at once (probably only the second one at the bottom of the frame is an actual minibuffer?), see picture below:
The two input areas is not what we get when the same is invoked in GUI rather than TTY (and not what the TTY behaviour used to be):
(In terms of how it's actually created, I guess that's in one of vertico / posframe / vertico-posframe).
Cheers,
Len.