Presumably a hit on any of the "before" keys triggers not just a replacement with the corresponding "after", but another pass through either a subset of the rep

Presumably a hit on any of the "before" keys triggers not just a replacement with the corresponding "after", but another pass through either a subset of the replacement list starting from the key that matched, or the entire replacement list from the top down; otherwise an identical pair wouldn't have a chance to be pathological.