After generating initial markers, the PreImagesTruncateManager code maintains an invariant that if a nsUUID is tracked in a truncate marker, it must have an updated sample of its last entry. 

The invariant is important for catching bugs in the process, but ultimately poses no risk for data corruption or data loss, and only introduces a very small change that pre-images inserted during initialization will never be truncated.

Given initialization has shown to take up minimal time, and a future pre-image insert into a given nsUUID will guarantee eventual truncation for the pre-images on said nsUUID, it should be safe to relax the invariant.