The failure mode was known (hence why it was considered out-of-spec) but it didn’t translate to an accurate risk assessment. It was deemed “in family” meaning it was known to be outside of spec but not a risk. Somehow, over many flights, something that was originally characterized as a safety risk lost its credibility as a risk. It’s not that it was unknown, but the understanding of the risk was changed. I wonder if STS-27 led to a false sense of confidence