It was also a little confusing for me but I think it might actually be intentional. The way I interpreted it was that Coro and Hakugin’s “mother” is actually a guy but because he was the one to give birth, he had the “feminine” role in the relationship and so people in their world just refer to all the birth givers as she/her/mother. Kinda like how in omega verse the male birth giver is called “mother”
Oh okay make sense, I thought it was just a typo but oh well...
I think the calling them mother is absolutely right. However calling them she/her is mostly on the translator in my opinion. In japanese they don’t state the gender of the person when they are talking about someone in third person, they mostly use “ano hito”(which I think they must have used in this one) that translates to “that person”.
The translator are quite confused since this story has not been updated for a long time. Coro's mother is a male right? They introduced the mother a guy before since coro's wife has a same situation as his