ToME: the Tales of Maj'Eyal

Small buggy: I killed a honeytree, as the last creature of a wave. At the moment of dying, it summoned a bear.
Killing the tree resulted in wave complete message, and I killed the bear while the countdown was ticking for the next wave

Heh, okay. It's not just summons made on the moment of death, it's summons in general.
When you kill a summoner as the last creature of those who originally spawned with a wave, the wave ends, regardless how many summons are still alive.