I've suspected this for a while, but have proven it for myself by keeping an accurate track of my Apple ID and Password, having written them down + kept a log on another system.
I went to the App Store to download the latest macOS Sierra update. While the auto updates worked well, signing into the App Store with my Apple ID did not work.
I successfully logged into https://appleid.apple.com with the same password. I re-checked all my details. I then changed my password in my Apple Account Manager at the above url and re-entered it in the App Store after I shut the App Store down and reopened it. However, it would not let me enter with the new (or old password).
Obviously I am perplexed, frustrated and annoyed by this.
Does anyone have any suggestions on how I can address this?