I would think that St. Louis trimmed spares would be mush after 3-2-1 at 250. I usually cook spares at 270-ish and they go for 2.5-1.5-30. I try to get racks that are between 2.5 and 3 lbs after trimming.
I would double check your thermometer to make sure that you are cooking at the temp that you think you are.
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