I totally agree with @M_Koniuszko. The whole tyre allocation system is poor compared to Kylotonn and real life.
I understand it's not necessary to change tyres at every service in the game but to be unable to re-use or rotate tyres on the car is just plain wrong.
It's not representative of the sport at all and ruins the custom championship mode.
However, in normal championship, the maximum number of stages you can have is 9 and the tyre allocation is fine.
But if you create a custom championship with more stages, it seems to still allocate tyres based on 9 stages.
I think that is the bug.