For my mappack, I have a large elevator room so I'm using linked_portal_doors to make sure they don't get in the way of the testchamber. It looks seamless from close up but once I get a bit farther away it turns white. What is causing this? I have all of the keyvalues for the linked_portal_door set to the default except the size.
I can upload screenshots/vmf if necessary.

I believe this is because worldportals have a certain render distance: if you're further away it won't render anymore and it will turn white. You can also add a small corridor between your elevator and test room. Add the worldportal at the beginning, then a small corridor, then a door and make sure the door closes as soon as you passed through, followed by the closing of the worldportals. That should solve the problem.

Hope that helped

I would say stay away from world portals as much as possible, they are very glitchy and should not be used unless something impossible is to be done.

I would have made/extended a hallway from your elevator so the chamber can fit.

I put a turn in the corridor and it works fine. Thanks for the help.

The main reason I'm using world portals is that another person working on the mappack is working on the elevator, so I really don't know what the final shape is going to be. I'm using a placeholder for now so that I can continue mapping. Once it's finalized, I might remove the world portal but I want to be prepared for the possibility that I can't remove it, for example, another corridor is in the space that the elevator is in. But yeah, I agree, world portals are a sub-optimal alternative.

Well, valve used the method of teleportation in their map sp_a4_jump_polarity. So not white outline. Play the map again and see how flawless it looks from far away. The elevators is with a small white corridor is outside the boxed map with a teleport point in the middle of the corridor.
Also if you want, I always use this method, put a world portal ent in the middle of a prop_testchamber_door, then another one at the other door and you're done no white stuff.

I thought the linked portal door, wouldn't draw when you were in a certain visleaf, and the game though you were around a corner or something, and thus wouldn't draw it.... it also depends on your graphics settings

In this case, it's not my graphics settings because all of them (except for shader detail) are set to the highest setting. Visleaves are a possibility though. I'll look into this.

I noticed this same thing when I had two linked portal doors close to the same area. If you are standing on one side of the door with the door open and can see the other door. it shows up solid white. As soon as I walked through the door to the other side it showed up as normal again.

So in my case it seemed as if the game wouldn't load the third area if you were still in the first aree.