Hidden bug in CLI tool only emerged when new adapters started returning real data

A software bug in a CLI tool called 'gx' went undetected for an extended period because both sides of a key data comparison were consistently returning the same placeholder value by coincidence. The flaw resided in a function called Engine::rehydrate_committed, which was responsible for rebuilding transformation data from a journal during undo operations but used a hardcoded placeholder instead of reconstructing the 'target' field. The bug only surfaced after two new adapters, 'fs' and 'git', were shipped and began populating the target field with real values, causing cold-process undo operations to fail with an internal error. An initial fix resolved the undo failure but broke redo functionality by incorrectly re-deriving a prediction for rows that were never meant to have one. The final solution distinguished between the two cases using existing data — specifically, whether a row had empty or non-empty parent records — requiring no schema changes.