Just taken my 599xx out to do the last 2 dailys and its nerfed. Normally use bumper cam and wondered why it was throwing sparks infront ,so swapped to chase cam and can now see the problem.
Over c 170mph the rear diffuser dissapears under the tarmac 
Thats some downforce! Car also has Renault 4 levels of body roll too.
Getting really desperate to stop us playing this game for whatever reason.
Which tune are you using? (never had an issue with my 599XX). Using the tune from Stomfire576 myself.
It was a snow tune,tried another now ,cant remember name, and much better.Still starts burying itself,but not until 260+mph now.
I cant honestly remember the last time i drove it on the motorway,so iām not sure how long its has been acting like this, on slower roads you wont notice it,so could have been like this for a while.