From Wikipedia: " Compressibility in pitot tubes of the time often resulted in exaggerated speed readings near the speed of sound, particularly in German equipment, which was adversely affected by supply shortages as the war progressed"

https://en.wikipedia.org/wiki/Hans_Guido_Mutke

In other words, it's the same problem that the piston-engined aircraft of the time had: Anomalous readings at high speed.