bbPress.org

I think this may do it – but I’ve no knowledge of mathml, other than a quick google

This code is untested, but add this to your child theme function file or use the ‘code snippets’ plugin

add_filter( 'bbp_kses_allowed_tags', 'rew_add_mathml') ;

function rew_add_mathml ($allowed) {
	$allowed['math'] = array() ;
	$allowed['mo'] = array() ;
	$allowed['mi'] = array() ;

return $allowed ;
}

you would need to add lines for other tags required.