For the other regression, here is a reproducer:

0. mkdir /tmp/{Test,retest}
1. emacs -Q --eval "(custom-set-variables                        \
   '(completion-category-overrides '((file (styles substring)))) \
   '(read-file-name-completion-ignore-case t))"
2. Type `C-x d /tmp/tes TAB': this completes to /tmp/test/
3. Type TAB again: Emacs dings and shows "[no match]"

If you repeat this recipe in emacs-30, then after the TAB in step 3, this
completion is shown: /tmp/Test/. and typing TAB again pops up the
*Completions* buffer displaying the two completions ../ and ./

While the result in emacs-30 seems suboptimal, since it omits the
possible completion /tmp/retest/, still it's better than "[no match]".

A workaround for both emacs-30 and master is to enter at step 2 `C-x d
/tmp/*tes TAB'. This shows the completion /tmp/*test/, then hitting TAB
again shows /tmp/*test/., and hitting TAB a third time pops up the
*Completions* buffer showing the four completions Test/../, Test/./,
retest/../, and retest/./

Steve Berman