Because Obviously you do not realize that Double the Resolution is the same as 2 screens each rendering simultaneously.
Regarldess. What’s 80% of 2 screens (at the res you shared)?
Still way more than your avarage user’s rendering.
Sure, the engine is broken, ain’t nothing new about that. The 80% is probably just The specific resolution above which it breaks. Which is higher than 4k, so epic won’t care.
If it were up to them, youd be rendering at 400x200 still.