It may be challenging to setup multiply SSH credentials to access different Git repositories. At least on MacOS it keeps losing which key file to use authorize against Git server. Linking it as regular in the ~/. ssh/config does not help in a long run.
Nice and bullet proof workaround is to define key file in the git repository’s config file (.git/config in a root of repository). Add these into is and all will just work:
[core]
sshCommand = ssh -i ~/.ssh/git_key_file