this is my own issue which is a specific case:
On linux:
my friend gave a laptop to me.
I had a same issue on my friend's laptop that he gave it to me with his github credentials. I did change my username and email in the git config --global and I send a PR, but I didn't see the compare and pull request button. I
I asked him and he was able to see the compare and pull request button. so I send a PR and he saw the button which it was weird.
I remove his ~./ssh/id_rsa.pub and ~./ssh/id_rsa public and his private keys, and replace it with my own laptop public and his private keys. and it worked.
it's like when you login to github, github saves your public key.
so I was able to see the button again.