Since JS is single threaded, they are basically keeping track of which component was last called for render - that way they know useState() calls attached to which components. If the order changes, then they can't keep track of the different objects tracked by useState. So treat it like you would class state - all fields are always there, no matter which functions or conditional branches you take - hence - do your initialization above all your conditionals and loops.

Edit: And yes, slight concern of course, it's the got some rough semantic equivalents to a thread-local in Java, albeit in Java the danger exists downstream, this danger is only exhibited during initialization it seems.