Hey @oodavid , that sounds like a bug I posted in 2017, here-
@nealwhite was kind enough to properly post it as a bug report;
So far no solution. Probably a good one for you to re-raise.
Incidentally, I haven’t found any workable hacks, either. If I have e.g. 10 domains on a site, and they all have to be separate, it appears somewhat random which domain appears in the sitemap.xml. However they all get the same one, regardless.
I’ve tried publishing all domains, and then just publishing ONE domain by itself, in the hopes that this would override all 10 sitemap.xml to contain the domain I want. No dice.
Your only option may be to copy your sitemap.xml, search-and-replace your domains, and then paste it into your site config explicitly.