As a mixed race person from a poor country who is usually interpreted as "white" but also grew up poor, I completely sympathize with you. What you are dealing with here is a combination of a word being used with two different meanings, and virtue signaling. You are literally African, as you were born in a country in Africa. So you are correct. However, "socially" (i.e. from a virtue-signaling POV) the word "African" on the form is probably meant to be interpreted as "black."
Now, maybe, racially (since you say you are mixed-race) you are indeed partially "black," and, technically, you are allowed to identify yourself as "black" if you have such heritage in your recent racial background. So you are still correct. However many white people (that is to say, people with no identifiable black ancestors in their recorded family history), feel that if they "read" you as white (i.e. if they, on the basis of no information but what they perceive in a conversation with you, ASSUME that you have no black ancestors), then you have no right to identify yourself as black. This is because they believe that such a practice is anti-racist. (You and I could have a whole private conversation about the implicit racism of white people telling mixed-race people how we have a right to identify, but you can't talk about it in public.)
So what has happened to you is that you intended your interactions with this group to occur on the basis of factual information and they instead interpreted your interactions based on social information.
My basic point is that this isn't really about ADHD. This is about other people virtue signaling at your expense even though you are the one in the right. (Essentially, it's about a microaggression.) It's possible that your ADHD makes you less willing/able to accept this sort of thing silently because your emotional lability is a bit higher. But you aren't wrong. What you are, however, is excluded from the group, which is not the outcome you wanted. So, going forward, what you need to do is decide whether being included in groups is worth keeping silent when you encounter these sorts of situations, or if there is a way to handle these situations that nets you the outcome you want (being in the group).
Sorry that it's so frustrating. It's not fair but sometimes you have do illogical things so that you can focus on doing what helps you thrive.