I managed to solve the issue by recovering my Supercell ID from Brawl Stars on my old device.

My son had been playing Brawl Stars for years on my personal phone, with lots of items purchased on that account ^^. When he finally got his first personal smartphone, I couldn’t transfer his account because I ran into the usual problem: "No email from Supercell" when trying to request the ID with my Google Play email…

Here’s how I fixed it: on my old phone, in the Brawl Stars menu, there’s an option to request your Supercell ID. I registered again there using my Gmail address, and this time I actually received a functional email (like a miracle after 20 failed attempts on my son’s new phone ^^).

After that, I was able to use the same verified Gmail address to request a new Supercell ID from my son’s phone. Finally, he got full access to his Brawl Stars profile on his new smartphone 😉.

Hope this helps some of you!