What's the output of:I had the same problem... After build php it's auto change to prefork. But after rebuild_confs it's change to selected one (i've got selected Event prefork in directadmin config).
I can't rebuild/update php becouse it's always change MPM for apache. Any solution? Mod_ruid2 disabled, enabled http2 (debian 8, i just update openssl to 1.0.2u).