If you target someone valid for Retribution’s instant kill while Immovable Object is up, Colossus will be downed regardless of his health. I targeted Storm with less than 40% health, the kill was redirected to Colossus despite him being at full health.
This happens with Deadpool too. Get any enemy down to 40% and fire off retribution. It kills Deadpool even if he’s max level and max health.
The Deadpool bug is supposed to be fixed in the upcoming patch; hopefully that fix will get the Immovable Object bug too.