Some blocks in debug world don't render properly. From some testing, I found out render distance has an impact on the flickering blocks, but the other rendering issues, the end portal,  persist. I don't know if this is exclusive to debug world or if these rendering issues are possible in a normal world. I did go into a normal world for a bit and didn't see any of the flickering issues. The end portal, however, does seem to still render incorrectly in a normal world.

Some videos demonstrating the issues in debug world:
https://youtu.be/zytoD_r-nRI
https://youtu.be/u12aqTPUvY0
https://youtu.be/uTY8dWzxL5E