Update i just played the game first run it didn't spawn, second time i landed before the ramp to jump to gap and explosive boxes before the missing portal and i let it slow motion while it gave me the RB to speed boost (using an Xbox controller) so it seems like i was going too fast for the portal to load in, so if you have this issue try slowing down for the section where you are having the issue... ideally using the tip to speed boost over a jump/gap as it puts the game into slow motion

I have the same problem.

New Update for New Game + (or whatever its called in this game)
Bug is still there and i can't do the fix i used on my first play through of the game.
If you have the game doing slow motion when riding the Speetle and approaching a jump that is how i worked around the missing portal, i also had all graphics options turned off and had it set to very low, that didn't work on it's own it's only when i let the game do slow motion for as long as i could that's when the portal worked

However as i said in NG+ you don't get the button prompt, that puts the game into slow motion, so i can't complete the Time Trial.

So there is only one way to get it to work that i know of and you can't do it in NG+
I don't know if the turning graphics options off and setting it to very low is important, but it doesn't fix the problem on it's own as i tried twice in NG+ and it didn't work.

It needs a patch i think, or i need to upgrade my computer, as i think its a loading issue
On my first play of the game the time i got it to work using the slow motion to let it load i noticed a gas tank thatch floating in the void's textures load and after that i did the jump and the portal spawned.

so yeah i think its just the game having an issue loading on some computers.

I can think of a way it could be patched but i can't see anyone caring enough to do it, method 1 to patch it is just optimise the games loading times for the race, as its the only time i have trouble with the games loading (once i used NVIDIA's optimisation) well i also have issues sometimes with the loading when you get sucked into portals most noticeable at the start of the game and especially in the first Nefarious Juggernaut boss fight in front of the statue in Nefarious City, method 2 is to change how the race works slightly, like have all portals spawn in to start with but maybe just hidden in terrain or something.

I don't know if there is a way to turn back on the button prompt slow motion in NG+
you know when you approach jumps on a Speetle (for an Xbox controller) it comes up saying 'Press RB to speed boost' (something along those lines)

I have the same issue. Also tried reducing graphics quality to very low but it did not fix.