For whatever reason, the top piece warped despite what appear to be basically perfect supports.
The supports are only lightly connected to the object, to make them easier to remove. The force being exerted by the plastic as it cools is stronger than the light connection to the supports, especially over such a large area.
Probably there is a draft causing it to cool to rapidly. An enclosure, even just a cardboard box around the print area, would help.
Xanthrax@lemmy.world 2 weeks ago
The heat from printing the end of those supports/ limbs. You need to reduce the heat as much as possible without clogging your nozzle.
Edit: A lot of people figured that out before me. Good luck!