That’s by the way why “third reich” is a really bad way to call it: Either call it “drittes Reich” (German) or “third Empire” (English), but mixing the two is just stupid.
If I had to guess this half-way nonsense is probably a result of the English not wanting to be associated with it, giving they kept talking about the “British empire“…
I mean, I’m completely down to misgendering or dehumanizing transphobes who do it first.
But that kind of situation aside, and cases where people do it without malicious intent (you example can also be because they think they misgendered you at first and are now trying wrongly trying to correct themselves) because they just genuinely guessed wrong aside, it’s almost always transphobia.
Now there is one other case that is a bit special: Intentional misgendering of people independent of their trans status: This can for example happen in pep talk in sports, if a couch refers to the members of the boy’s team as “girls” or when someone ironically refers to a group of men as “ladies”. This is not generally speaking transphobic, but it can be potentially be sexist.
To give an example: At the start of my previous job (before covid and before I transitioned), I was talking with my boss about alcohol and that I prefer cocktails. Apparently the same holds for her boyfriend and she refered to us as “the girls” over this. Nowadays when I tell that story, I like to joke that “if [her boyfriend] wants to have a private conversation with me, I’m definitely available for that”.