:bufdo executes its arguments with ignored Syntax event. E.g. try
:bufdo set ei?
to convince yourself.
When :bufdo finishes, it fires the Syntax event to activate syntax
highlighting for the last (= current) buffer. This clears the syntax
highlighting that you just defined for that buffer.
Suggestion:
augroup MySyntax
au!
auSyntax * syn region Region1 matchgroup=Highlight1 start=+(+ end=+)+
augroup End
When defining these commands in the vimrc, I'd also make sure this is
installed after other Syntax autocmds:
augroup Vimrc
au!
au VimEnter * call s:AfterPlugin()
augroup End
func! s:AfterPlugin()
augroup MySyntax
au! Syntax * syn region Region1 matchgroup=Highlight1 start=+(+ end=+)+
augroup End
endfunc
A surrounding :augroup command is not required, but a good habit ...
makes it generally easier to filter and remove autocmds.
--
Andy