I don’t have a
Formentor, I’ve got a Leon eHybrid, but basically the same powertrain. And yes, every so often, but randomly, it will start up using the ICE, rather than on battery power (this, despite it being designed to always start in EV mode). Typically this is early in the morning, especially if it’s been a cold night, and the pre-heat has engaged.
It used to happen with my Golf GTE too, so I do think it’s “normal”, yes.
i vaguely recall hearing it has something to do with the 12V battery being slightly low on charge, that is all. But I don’t know. Personally it doesn’t bother me, I just accept it as one of the (many) quirks of the thing!