A lot of it has to do with equality and countering bias. If you’re writing alt text for a photo or doing audio description for a video, then the recommended policy is that you either describe everybody’s race or nobody’s: if you only describe the races of minorities, and leave people to assume white when you don’t describe people’s races, then this plays into the idea that being white is the default and anything else is somehow “exotic” or “other” or whatever. By not describing people’s races you also make it harder for whoever you’re talking to to apply their own biases/prejudices towards the person you’re talking about. Describing people’s races excessively can also make it seem like you’re yourself weirdly fixated on that one aspect of their being, which would be reductive.

This being said, I can only assume that your manager isn’t some great anti-racist activist. They’re really just trying to cover their (company’s) ass, which is why they didn’t explain the rationale very well (they don’t actually understand it) and their behavior isn’t consistent (they don’t actually care). There is also nuance to all of this, too, naturally, since “colorblindness” is not true anti-racism, so there are cases where it’s actually better to mention people’s races/ethnicities than not mention it; and if certain features of appearance weren’t stigmatized/racialized, they’d be just as suitable for a short description as any other feature. The problem is that you cannot easily teach these sorts of nuances to people who haven’t actually experienced racial prejudice: white people will (deliberately) misunderstand the nuances or try to do borderline things. So you end up just getting these blanket bans and taboos on mentioning race that make the frank and actually important discussions about racism difficult.

This is how I understand it, at least. Bear in mind that I’m not racialized myself, but the other commenter — infuziSporg — is.