Thanks for that link. I found a second place where I had to tell (I guess) the theme what was the primary menu. I guess the first time I only told WordPress.
Unfortunately, my troubles are not over. BBPress says that if I have pretty permalinks enabled, which I do, the custom link /forums will go to the top of BBPress, the forum list. Once again, this works flawlessly in GeneratePress, and 404s in the child themes. It doesn’t matter whether I type it in or click the menu, now that I can. 404 either way.