If you replace the mob with mobspawn trigger it no longer counts toward game limit - upshot of this is that you end up with a massive number of mobs after a period of time.

Yeah :D Fixed in dev build 48, refer to commit d5b0a98.

There is now a config option "custom_spawn_limit" defaults to 150 & only applies to the mobspawn trigger (so even if the limit is hit drops from another mob, block or the odd command will still work.

This explains a lot haha, I kind of figured this was happening when I ended up with 6000 mobs on one map.