It happened to me in other car like fox, its hard to reproduce the "fail", but i found a nice set to do it with F08:
I'll attach replay
I have got not a good english so... the replay will explain the problem itself. By the way, if you put bot cars with that setup, they cant gear up
In the video the problem appears when i change 3rd to 4th or 4th to 5th

Note: I'm using mouse+keyboard, so gear up its done just pressing the key, in the replay sometimes i use the clutch or not throttling.
Try the car by "test drive"

rev bouncer?

Nope

I tested the car by test drive , and it's really weird.

You can only change gear on 4000 rpm

when arrive to 5 or 6 , you cant

I have also experienced this problem with FOX and FO8, but not as bad as you. An example of this is in the replay I attached approximately at 47-48 seconds.

Ye i saw

But why happens this?

Never happend to me tbh

I guess it's the same bug that sometimes don't let you change from 1st to 2nd while your tires are spinning.

I belive it has something to do with how much torque is generated on the engine and how much is transferred into the ground. I think it works that way because this never fails at redline, and it's the first time I see it failing without tires spinning, but this setup is oval-oriented, so may be a extreme situation that also causes the bug.

Well Whiskey.

But as ImudilaSkyline showed us.

Doesn't happen only on oval , he had some issues too on FOX @SOUTHCITY

What about that?

Quote from Whiskey :

I guess it's the same bug that sometimes don't let you change from 1st to 2nd while your tires are spinning.

I belive it has something to do with how much torque is generated on the engine and how much is transferred into the ground. I think it works that way because this never fails at redline, and it's the first time I see it failing without tires spinning, but this setup is oval-oriented, so may be a extreme situation that also causes the bug.

Nope the set is not oriented for oval, look at the camber. It's a normal set with 20% restriction, so the car is slower.

Don't ask me for the setup because i have deleted it, just because i thought we had enough with "test drive"

ai cars also spin at last corner when going to pits in some fern bay tracks.

Quote from josemspain :

It happened to me in other car like fox, its hard to reproduce the "fail", but i found a nice set to do it with F08:

You are using an FJR set, with at least 20% of intake restriction and that is known to cause problems in shifting.
I suppose you're using FOX with intake restriction too and then experience same problems?