So, with the automatic sentry summoning added relatively recently, it seems that the code which desummons and resummons the sentries to make them "follow" you is running when you haven't assigned any sentries to an auto-summon slot, so it is causing manually-summoned sentries to be de-summoned when you move around.
So, with the automatic sentry summoning added relatively recently, it seems that the code which desummons and resummons the sentries to make them "follow" you is running when you haven't assigned any sentries to an auto-summon slot, so it is causing manually-summoned sentries to be de-summoned when you move around.