it should be in your MS online account as someone wrote, but in case of, I always save it on a USB key, hidden somewhere. You can also print it, or take a picture of it with your phone. Because there is no way to get it back.

Because they force you to use online accounts now, you can get it from the registered account via the Microsoft account page.

In your Microsoft account: Open a web browser on another device. Go to https://account.microsoft.com/devices/recoverykey to find your recovery key.

The automatic encryption and subsequent backup both took place because you were using a Microsoft Account

LUKS don't use TPM

There's an extension that can unlock LUKS drives using the TPM, but by default it does not do that, and probably that extension isn't installed either