emacs -Q
   M-: foo M-< C-x C-x

In emacs 26, point returns to the end of the minibuffer, but in 27 and
28.2 a bogus "No mark set in this buffer" error is reported.

I think the first 'when' in minibuffer-beginning-of-buffer is supposed
to be 'unless'.

Another issue is that this:

   emacs -Q
   M-: C-x C-x

correctly reports "No mark set in this buffer", but this:

   emacs -Q
   M-: t C-@ RET
   M-: C-x C-x

does not.  This does no harm, but logically the mark should probably
be unset when a new minibuffer is created.