Invalid refresh_token

Hi there!

We have an integration app and one of our clients is having a refresh_token issue.
Its access_token is invalid and when our app tries to refresh it using the refresh_token, we are receiving the message that the refresh_token is invalid as well.

Wasn’t refresh_token supposed to be permanent?

I am using OAuth authorization:
The error message is: {“success”=>false, “message”=>“Invalid grant: refresh token is invalid”, “error”=>“invalid_grant”}

Direct messaged for further info.