Stephen Berman <stephen.berman@gmx.net> writes:

> 0. emacs -Q
> 1. M-x customize-option RET read-buffer-completion-ignore-case RET.
>    Toggle to "on", set for the current session.
> 2. Type `C-x b RET a', then `C-x b RET A', then `C-x b RET ahjk', then
>    switch to buffer *scratch*.
> 3. C-x b a TAB => Emacs correctly reports: "[Complete, but not unique]"
> 4. TAB => *Completions* buffer contains, as expected: A a ahjk.
> 5. C-x k ahjk RET.
> 6. C-x b a TAB => Emacs incorrectly reports: "[Sole completion]".
> 7. C-x b A TAB => Emacs incorrectly reports: "[Sole completion]".

(This bug report unfortunately got no response at the time.)

I tried this in Emacs 28, and I got: