imglower | Tubba Blubba Out of Bounds @imglower | Uploaded November 2019 | Updated October 2024, 1 hour ago.
If you are entering the room while chapter 3 is triggered the cutscene with yakkey begins and you are warped to hide behind the bed. If you do it before you open the chest, tubba is not there because he was spawn and is now on his bed, however yakkey is out of bounds in the same position as tubba is in this video. After obtaining the key yakkey is still there and tubba is not because he's still chasing you. Upon completing chapter 3, if you come back you can do just what I did in this video; fight tubba out of bounds.
BTW yes he is still invincible and you can't kill him :(

TR;DR
Do this after completing chapter 3 to fight tubba, otherwise he isn't there.