>>> Michael, please check whether it fixes your
 >>> use case.
 >
 > That seems to be the case, yes.

Probably good to know.  Meanwhile, please try the attached patch which
should "cure" the underlying problem (it's the same as for Bug#34138).

It does not cure another problem I found this way based on your
scenario:

   With emacs -Q do M-x shell, C-x 5 2, and M-: (message "A\nB").

Here with emacs 25 this grows the minibuffer window and displays the
two lines

A
B

in it.  With emacs 26 it shortly resizes the minibuffer window and
then displays "A\nB" in it.  With patched master it displays

Eval: (message "A\nB")

with the cursor at the beginning of the minibuffer.

I have no idea yet what's causing these behaviors but so far the
presence of an external procees or shell looks like a necessary
prerequisite.  Ideas welcome.

Thanks, martin