I don't register INSTANCE_ENCOUNTER_ENGAGE_UNIT, neither do the current oUF version. And it updates properly, meaning it also shows all boss units. There are 9 UNIT_HEALTH_FREQUENT events between 2 UNIT_HEALTH if I recall correctly. If UNIT_HEALTH_FREQUENT is a hidden polling, why would Blizzard not just register for it but rather poll it in lua in
UnitFrame.lua and the like?
I'll try to test Omnotron Defense System as INSTANCE_ENCOUNTER_ENGAGE_UNIT probably plays a role there.