https://bugs.winehq.org/show_bug.cgi?id=50896
--- Comment #3 from Esme Povirk madewokherd@gmail.com --- I sent a patch for this: https://www.winehq.org/pipermail/wine-devel/2021-May/187654.html
I don't really have a way to test it, though. As you've observed, Wine doesn't complain if the font family is used after being deleted, so I just have to hope I've implemented this correctly.