Help says "this works":
:augroup mine | au! BufRead | augroup END
but it doesn't (augroup "mine" still active afterwards), thus it can't be
intentional and you found a bug.
Also not working:
:augroup mine | au! BufRead *| augroup END
" `*|' is taken as autocmd-{pat}, `augroup END' becomes {cmd}
:augroup mine | au! BufRead * | augroup END
" uses `| augroup END' for {cmd}
Weird that it went unnoticed for so long, probably because most people
instead use
:au! mine BufRead
I think Vim should make anything after `:au' part of the command.
`:au' is already listed under `:h :bar' without further notice.
--
Andy