bug#69738: [BUG] rmail-mail-new-frame doesn't delete the new frame after composing the message on Emacs 29.2

[rameiko87]

For brevity I don't include the steps for C-c nor for C-s but only for 
C-d and C-k; but I can assure you that the case of C-c would yield the 
same result; I wonder what the desired result would be for C-s instead.

Steps to reproduce:

emacs -Q -nw
M-: (setq rmail-mail-new-frame t)
M-x rmail
m
C-c C-d

alternatively, use C-c C-k on the last line.

Expected result:
The frame is deleted (as the manual specifies at the end of 34.10).

Actual result:
The frame is not deleted.