OK I found it myself : https://us.motorsport.com/f1/news/were-russells-worn-tyres-the-cause-of-his-spa-f1-disqualification/10640096/

“We expect that the loss of rubber from the one-stop was a contributing factor, and we’ll work to understand how it happened.

“We won’t be making any excuses though. It is clearly not good enough and we need to make sure it doesn’t happen again.”

The missing weight of 1.5kg may sound like a lot, but we are only talking about 375 grammes from each corner.

And as Pirelli head of F1 Mario Isola has explained, that is well within the boundaries of how much a worn tyre loses.