I’m a German now living in the US. I was always befuddled when Americans asked me where I’m from and then exclaimed “I’m German, too!” No, you are not. You are very clearly American, lol.
This rarely happens now, but did quite a bit in the early years. That’s also how I learned about the “German” Christmas pickle, which is unheard of in Germany.
I actually once met a German American (German mother, probably also German passport, and spoke German) who was clearly unfamiliar with German norms. We were introduced, he asked me how I was (in German), and started talking to someone else before I could answer. This is rude behavior by German standards, but polite by American standards.
The cultural competence gets lost very quickly, even when your immigrant parent teaches you the language and, presumably, the traditions.