An induction stove generates AC magnetic field. The AC field induces eddy current in conductive materials, they do not need to be ferromagnetic (such as your Al foil). In the cooking pans or pots there is a ferromagnetic material exhibiting large power losses (wide B-H loop), these losses play dominant role in heating up the pot or pan. The induction stove heats pans (with ferromagnetic inserts) using both phenomena, but a non-magnetic material will be heated only with eddy currents.

Foil can trap heat or melt, resulting in damage to the product and a shock or fire hazard. Electric radiant cooktops: Do not use aluminum foil on or near a radiant cooktop burner. ... Gas ovens: Never cover any slot, holes or passages in the oven bottom or cover an entire rack with materials such as aluminum foil.

An ultrasonic frequency induction surface cooking unit heats aluminum foil and other thin metal utensils placed on the cool cooking surface. For optimum heating the aluminum foil has a thickness of 0.5 mils. A uniform heating distribution is obtained by preferably using a rectangular induction heating coil with several series-connected elongated coil sections, or by varying the metal thickness to graduate the energy acceptance. Frozen convenience foods can be defrosted in this manner, and the aluminum foil can be wrapped about the food and shaped by the user into disposable utensils using a set of molds. Disposable foil cooking obviates the clean-up and storage problems of pots and pans

Details here

https://www.freepatentsonline.com/3786222.html

Why does aluminium foil get hot on an induction cooker, and why does it stop when you put an aluminium pan above it?

Since you should never put aluminium foil on an induction hob, but I would imagine in that scenario that the current gets shared between the pan and the foil, so that most of it ends up in the pan. The magnetic field from the pan will to a fair degree oppose that of the cooktop and the current through the foil will go down sharply.