Summons are dismissed upon the summoners' death, so it'd be fair to make them dismiss when subdued also. Otherwise, in pvp it is almost a punishment inflicted on the subduer for not being on full damage instead.
It also makes sense that the summoner would lose their focus on the creature when knocked down.
You can knock out the character. I think the command is "/c subdual bo", but I don't know pvp much so someone may need to correct me.
I thought this already happened? I subdued someone today and their summon was unsummoned.
I think if you hit them hard enough the summons are dismissed. Similar to how if you keep on attacking a subdued person, they'll slowly start being unequipped of their weapons, armour, etc.
Yes, I think they need to sometimes just be hit again or hard enough to be brought into a higher "tier" of being subdued.