I honestly have no idea what's happening here, please help. It was perfectly fine before I opened the portal, then it messed up. FastRender and SmoothWorld are off, if you are wondering.

Apparently, when I look away, that portion disappears.

That's because the "Debug View" in BP's config has been enabled.

Did you enable it and forgot about it or are you using a mod pack which had it enabled? I'm genuinely curious cause there have been quite a few reports about it.