Edit : I found another solution, I was working on a project born on 4.26.1 that was converted to 5.0.
I tried re-creating a new 5.0.3 project from blank and I migrate everything from the old project, same project settings and all, and now the problem is gone.
It seem that in the newer version of UE the Flag is fixed…
I suppose Unreal create the files to sign the apk just at the creation of the project and it never updates them?
Anyway I solved it , hope it helps