With same grip level?
What on other tracks? Try some Kunos tracks, and mine like Chang (you can find it here). It has same stuff as Okayama. But I am almost positive its not track`s fault. You are actually the 1st one who reported this kind of issue. But I will try to help in any way possible.
PS: I did countless laps at Okayama with JT5, but never seen anything like it. So it must be connected with VR somehow. Does anyone else experienced something like it as OP?
And on other tracks (Kunos, or Chang)?
I wasn't outright blaming the track as it's the car that makes the difference. But there seem to be some difference in tracks too. Kunos tracks seem to work. I tried Brands Hatch and Mugello with JT5 - track markings and rubber are there.
I also tested the Chang (great track btw) as you suggested and it's the same as Okayama, no track lines (starting grid is there though) or rubber. I also tried LilSki's Watkins Glen and there's slight difference: it loses rubber but not the lines on the side of the track.
Track is set Optimum at CM on all these tests. And I figure that wouldn't make the difference as also track lines are lost.
Interesting problem. Would be nice to sort it out as Okayama is one of my favourite tracks and Super GT would naturally be just the car to drive there.