The basic (original S-1) design doesn't join strongly in the center, and needs a stiff covering, such as doped silkspan (or silk). Film coverings allow it to flex enough that after a time, the outboard wing breaks away in flight.
One thing I omitted mention of, it's so basic that all the old-timers know it, both silkspan and silk have a "grain" that should run parallel to the wing's span for best strength. For silk, it's in the "long" direction, with the selvage edges, while the paper has no similar giveaway. You must attempt a tear near a corner. The direction it tears neatly & straightly in is the grain's direction.
My mentor was never a big fan of Ringmasters, though he claimed to have built a half dozen Circus Kings, which were clones (also Matt Kania) of the Ring, just better looking in his eyes. He'd say something like he'd never seen a Streak shed an out board wing on its own, compared to having seen a dozen Rings do just that.
When I began building my own S-1, he suggested gussets at various critical corners, such as from the center rib to the TE, and center rib to LE, plus twin gussets from the second outboard rib (end of center sheeting) to LE/TE, and/or adding curved segments to the sheeting that functions as gussets, although on the outside of the structure. None of that keeps the wing from flapping if the center joins aren't good, or if the covering is flexible, but they keep the outboard wing from hinging on the LE's joint in a rough landing.