Dirty track tyres

I have done dozens of track days at this track and noticed car not turning in as usual and oversteer. Also intermit vibrations. About 3 secs a lap slower. Checked pressures. All ok. Noticed a lot of rubber build up across the tyre. (MPSS on 430).
The pic was taken a few drives after the track day. The rubber deposit was twice as bad at the track, btw. It was everywhere.
We were in a pm session and the am session was race cars, radicals (also world time attack cars so prob very soft tyres) shredding marbles everywhere.
Never seen it stick like this before. It usually cleans off as you circulate. It really has become part of the tyre and cannot "pick off".
I am assuming this was the problem. My contact patch must have been very messy. How do you handle this at the track? Are there files for this? Was thinking about a burnout for the rears but what about the fronts?
Like I said, I just have never seen it do this before, so will blame the time attack cars.
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I think so because I have done same track, same tyres before without ever seeing this much excess rubber and I have previously been able to pick any excess rubber off.
Car felt terrible all day. Could not shake off those excess beads of rubber.
Come to think of it, a few other drivers were also complaining of unusual lack of grip but I didn't take note of their tyres.
Will try the Sport Cup2 next anyway.

 

Thanks for the info. Should have thought of that!
As I said, the rubber normally comes off on clean laps but this one particular day it just would not come off so I will try the heat gun.