Surgery in the tendon, and this is the second time in his life. This was the reason because in 2020 Sondre ran on track with racing shoes, not for having some advantage.
The funny thing is that he bettered the European Record of 25 km on track in June, using Vaporfly (and the record was validated), but 4 days before his attempt against the European Record of one hour WA put the rule that it was not possible running on track with racing shoes, but with spikes only.
This was one of the most stupid and unfair rules ever in athletics, thinking that the type of shoes depends on the surface only, and not on the distance you run, too.
Running one hour or longer distances on track with spikes is the best way for destroying tendons and producing other kind of injuries.