I think maybe moderators cannot see that board because they are normal "User" .
So I picked up one moderator , cancel his "User" group , and he can "Only" see that hidden board and his moderating boards. He cannot see other boards.... This is so terrible.
So I enable his "user" group , and everything is OK now , he can see normal board and moderator-only board (he is still the moderator of that hidden board).

I think the permission is buggy in 7.3.1 , and need to be checked thoroughly.

English is not my native language. I try my best to express my thought precisely. I hope you understand what I mean. If any misunderstanding results from culture gaps, I apologize first.