A Document Lifecycle has an Entry Action set to supersede previous versions when the document is approved. The Entry Action is not completed on some documents when they are moved to an approved state.
The previous version is not in a Steady State. The Supersede Previous Versions entry action only applies when the previous version of the document is in a Steady State when the action is triggered.
Document 1 works to the Approved for Distribution State with an Entry Action on the state to Supersede Previous Versions. The Previous Version is also Approved for Distribution, which is a steady state. The action completes on this document.
Document 2 is set the same way, but the previous version is set to Draft. The Draft state is not a Steady State, so the action does not complete on this document.
Have a step in the Approval Workflow that ensures the document, that is about to be moved to Approved for Distribution, is set to a Steady State somewhere before the document is approved.
Vault Document Lifecycle Documentation: Define Document Entry Actions