This is probably not a bug from autobuilder but rather from Buildbot itself.

I found this discussion along the way during my discovery - https://lists.buildbot.net/pipermail/devel/2006-October/001555.html

Hence this is what could happen, i gave up on permanent solution and resolve on my own workaround.

$ buildbot restart yocto-controller 
never saw process go away

$ ps -ef | grep twistd

user 6577  1522  1 10:19 ?        00:00:07 /usr/bin/python3 -c from twisted.scripts import twistd; twistd.run() --no_save --logfile=twistd.log --python=buildbot.tac
user 7536  4318  0 10:28 pts/2    00:00:00 grep --color=auto twistd

$ twistd-kill ~/yocto-controller restart


# Simple script to forcefully killed the reactor still running on the background.
#!/usr/bin -xe

BUILDBOT_PATH=$1
BUILDBOT_ACTION = $2

kill -9 `cat $BUILDBOT_PATH/twistd.pid`
buildbot $BUILDBOT_ACTION $BUILDBOT_PATH