Sorry, my answer was confusing. The drawn solution is not correct since the formula is false on both structures, so that the two structures do not distinguish between the formula.

If a is always true, then [a WU b] is always true as well, and then [a SB [a WU b]] becomes [a SB true] which is always false, so that [a SW X[a SB [a WU b]]] becomes [a SW X false] which is also false. So, the first structure is not satisfying the formula.