You must also enable teambalancing for the bm gametype:
<!-- specify which gametypes to autobalance - separate with space or comma -->
<!-- FreeForAll=dm, TeamDeathMatch=tdm, Team Survivor=ts, Follow the Leader=ftl, Capture and Hold=cah, Capture The Flag=ctf, Bombmode=bm -->
<set name="autobalance_gametypes">tdm</set>
Default its only tdm (teamdeathmatch).
Be aware that teambalancing in a gametype like BombMode will kill the player when balanced, you may not want that to happen.