+1, I don't think it's a cable tension issue since all the other gears trim OK. It could be a kink causing the cable to hang at just one point, or possibly a hanger that isn't square.
Also whenever I see an RD that shifts erratically only on the lower (larger) sprockets, I double check the "B" screw adjustment. If the RD is too high the Jockey pulley can be too close to or touching the neighboring larger sprocket making that shift problematic. Moving it down to improve clearance usually solves the problem.
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