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Isn't it satisfied S2 but NOT S1?
X[a SU b] holds on the single path of your structure, but [(X a) SU b] does not hold there, so that you are right. I think that this alternative solution has already been suggested before in q2a (see https://q2a.cs.uni-kl.de/3448/september-6-2022-problem-8-f).