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.