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@Paul Beuk and @John Carr, thank you for suggesting they could be gall midges. It might indeed be the right path.
It is all quite puzzling, since no one, to my knowledge, has ever seen these phenomenon in the past, until last year. This makes me think it could be some recently introduced species.
I am absolutely not an expert here. Tried to find similar cases online and have searched into some European entomological forums, without success.
I have read on https://bladminee...domyiidae/ that larvae of Contarinia sp. are able to jump. They also pupate on the ground. Maybe this could be a good start. I should look for Contarinia species developing on Cupressaceae. But it's such a large genus...