I'm using optifine so this might be a rendering bug, but here's a video:
https://dl.dropbox.com/s/jwm279qoqr0cce2/2018-07-02_21-46-15.webm?dl=0

The rift has gotten huge due to logarythmic growth (as far as I know it is intended) and is shaking around all over the place. It looks more like a flat billboard rattling around at mach 5 than a rift. It's also not centered on its origin block, instead the origin is at the bottom left of it?

I tried removing optifine, but it's not fixed. I'm assuming either there's a conflict with another mod or something happening with my GPU?

My version of the mod is the latest from curseforge.

My modlist:

I really hope it's not a conflict, because I have (according to forge) 84 mods active for this map. I can try removing everything but dimensional doors and viewing this area again?

Yeah, here it is happening with everything disabled but dimensional doors:
https://dl.dropbox.com/s/b8v12tayndplf12/2018-07-02_22-46-02.webm?dl=0

Not sure what's going on. Is the version on curseforge old?

My specs also, from dxdiag:

Yeah, that's definitely not what it actually does or looks like. See if removing Optifine fixes that, but we haven't gotten reports of something like this before