Yes, you are not tripping: many Dutch and Flemish people do react defensively when you say Afrikaans is similar to Dutch. This is not because they dislike you or South Africans, but because the comparison often feels reductive to them, touching on sensitive points about language identity, colonial history, and the fact that Afrikaans has evolved into a separate language with its own grammar, vocabulary, and cultural weight. They want to acknowledge the shared roots while firmly establishing that Afrikaans is not just "Dutch lite."
Linguistically, you are correct that Afrikaans is very closely related to Dutch, especially in vocabulary and basic sentence structure. It is often called a daughter language, and a Dutch speaker can understand a lot of Afrikaans without study. However, the differences matter: Afrikaans simplified verb conjugations (no "heb, hebt, hebben" distinctions), dropped grammatical gender, and developed unique words and pronunciations. For a Dutch person, hearing "Afrikaans is just like Dutch" can feel like someone saying "American English is just like British English" but with an even sharper historical edge, because Afrikaans was shaped partly by colonial Dutch, but also by Malay, Portuguese, Khoisan, and other African languages. The defensive reaction often comes from wanting to respect that distinct evolution.
Culturally, there is also a layer of sensitivity around the history of Afrikaans in South Africa, including its association with apartheid and Afrikaner nationalism. Dutch people may feel uncomfortable with an overly casual comparison that glosses over this. Additionally, Flemish speakers (in Belgium) have their own sensitivities: they often feel their version of Dutch is sidelined or seen as a dialect, so when you compare Afrikaans to Dutch, they may worry it reinforces a hierarchy where "Netherlands Dutch" is the standard. As for "welterusten" (good night), it is indeed perfectly normal in the Netherlands, but in Belgium, "slaapwel" or "goeienacht" are more common. And yes, in Afrikaans, "poes" is a vulgar word for female genitals, not a cat, which is a classic false friend that can cause awkwardness.
For your next steps: keep being curious and respectful. When talking to Dutch or Flemish people, acknowledge the differences explicitly: say "Afrikaans and Dutch are very closely related, but they have evolved differently." This shows you understand their perspective and opens a more balanced conversation. If you are learning Dutch, focus on the standard language (choose either Netherlands or Belgian Dutch depending on your goals) and be aware of false friends like "poes" and "welterusten." Use resources like online forums, language exchange apps, and grammar guides to navigate these nuances. The key is to appreciate both the connection and the separation.