This lead me to the solution! Thank you!
Turned out some sloppy coding wasn’t shown by VS but troubled the Build. I had some functions with local variables that hid member variables. I do that sometimes for clarity when I return the value of the local variable into the member variable anyway and it has worked before but this time I guess UE finally objected to that practice. Maybe it was just that the functions hadn’t been properly fleshed out yet (I was still laying out the structure of the class).
Anyway, I changed some variable names and suddenly everything works again!