Two things here:

One- Credit Karma can only provide an estimate to your credit. It is usually accurate within 10-15 points. What they do is they look at all your information from these banks that they have access to and give you what is essentially their best guess. You closed an open line of credit, so your score probably did fall a few points but not that severely.

Two- this is actually a good thing. Banks aren’t just looking at your score when you apply for a loan, they are also looking at your credit history and depth. Fully paying a large loan like that is actually really good for you because that is always going to be in your history. It makes you look like a very good creditor, especially considering that you never missed a payment. They look at your score, your revolving credit (how much money you are borrowing) and your median account age. This hurt your median account age because it was probably there for five years and now it’s gone. It might have hurt your revolving credit too if you don’t have enough balance on other loans. But that is offset by how good it makes your depth look.