App Engine and Firebase Hosting in One Domain

I had the same issue and contacted Google Cloud support, here's their answer:

App Engine doesn't need to have the A records on the root domain if you are only serving from a subdomain. App Engine should work properly for you with just the one CNAME on subdomain.example.com.

I found the solution myself, here you go!

On the fourth step in this docs, i didn't add A records, just add AAAA records. Suprisingly, it works! :D