More data!
It seems this problem only occurs when both "Only retract when crossing open space" AND "Avoid crossing outline for travel movements" are checked. If either is checked or neither are checked, retracts off of that support are okay. But if both are checked then the retracts go wonky as shown above.
I'm going to try one or two of my other objects that have this issue to see if unchecking "Avoid crossing outline for travel movements" and leaving "Only retract when crossing open space" checked will produce similar results.