I suspect this is because the gelling sugar contains palm or sunflower oil.

Hello springtimer, thank you for your request. Jam tends to foam when it is boiled down. This becomes noticeable at a later stage, for example in the form of air bubbles or a grayish film in the jar, which is often mistakenly interpreted as mold. To counteract this foaming, we use sunflower oil in small quantities (approx. 0.02 %). Kind regards, Tabea

Aha, thank you