Yeah, unfortunately, it's a thing. I have literally seen it happen on feminist-y Fb pages and groups, I've even had a middle-age white woman at work complain about it to me, I guess because she saw that I'm an afab white latine (I work in a very pro-Tr*mp town, so I choose not to tell anyone that I'm genderfluid or dress that way for my own safety, so most people just see "white (assumed cis) woman"), about how "mean" and "hurtful" it is that "black women come up to her and tell her she's not cut out to raise black-mixed children". And on the one hand, I can empathize with the frustration of having strangers come up to you and tell you that you're a shitty parent because you're not raising your child(ren) in a manner that they approve of (which is something that happens to a lot of people that identify as mothers) BUT at the same time, in this particular instance, they have a point.
There are so many adults out there who have confirmed through their own personal experiences of growing up as a mixed child with a white parent (a white mother in particular) or being a black child adopted and raised by white parents, that regardless of how well meaning or whatever their motivations are for having a mixed/poc child, they often do not have the desire to raise that child to be aware of how that part of their identity affects/will affect their lives as they grow up or to be close to that aspect of their identity. There are white mothers out there who act sanctimonious about having a mixed child(ren), behaving as they're on some higher level because they're teaching their child(ren) to be "colorblind" and "not judge someone based on the color of their skin", but behind the scenes subject their child(ren) to a bunch of micro-aggressions as they grow up, often with passive-aggressive/catty remarks about their "wide nose", "big lips", "nappy, unruly hair", their skin being "too dark", and that often goes hand-in-hand with the white mother being resentful of the black father who she was only with to have "beautiful mixed babies" or to "piss off her conservative parents". I still remember several years ago reading a story on Facebook about a white mother cutting off her mixed daughter's braids because... ooh it's been awhile so I don't quite remember, but it was either because she accused the child's black father of "ruining her hair" by putting them in braids to begin with (a protective hairstyle for natural hair btw) or the white mother fucked up her daughter's hair by getting it tangled because the mother didn't want to bother looking up how to care for black hair, instead treating it the same way she did white hair (tl;dr - caring for and styling black hair is very different from white hair due to things like pattern, texture, thickness so you can't really do the same thing to it that you would white hair) and she cut off the daughter's hair... I want to say it was more likely the first version of the story involving the father because I believe it was him who posted about it online to vent his frustration.
Apologies for the long post. I don't want to appear as though I'm speaking for black women or black mixed adults who have experienced this. This is what I've learned from listening to these stories. I know you're not fond of recommendations for many things, so I apologize now for this, but on the off chance you choose to learn more about this, Kat Blaque has a video on it from her old JSYK (Just So You Know) series on her channel where she interviews an adult transracial adoptee who shares her experiences with this - the video in particular has text on it with a quote from her during the interview where she says "You can't love away racism", and T1J has a video called "Adopting Kids Outside of Your Race" that touches on it too. (Again, if you're interested in sharing this depressing learning experience.)