I’d agree early on with a newer character, but I already learned everything in the knowledge list since I’ve had my character since 2018. 
Ive also noticed some tavern patrons that I cannot interact with, although most I can. There doesn’t seem to be much rhyme or reason to it, although I have thought it might be a loading issue.
I had not learned it yet when I rescued her. I did end up just farming a fragment to get the points to learn it.
Just so I understand, you’re saying you did not already know how to make the bar and when you rescued a barkeep it didn’t have an option to teach you? If that’s the case then, yeah, that’s definitely a bug as you’re supposed to be able to learn it either way.
From my understanding and experience, if you learn it through knowledge points then the option to learn from a rescued barkeep doesn’t show up on the conversation list when talking to them as you already know it. And obviously, if you learn it from a rescued barkeep first then you can’t use points to buy it as you already know it.
Correct.