I have the Node revision module (diff) installed, and I have many revisions for a node, but the Revision tab is not showing any of them, even though the database table contains the data for those revisions.
What may the reason be?
A core issue related to languages was just fixed today: NodeController::revisionOverview() shows no revisions if node has no translation for current language
However, since you use diff, which replaces that patch and does its own thing, it might be the same or a similar bug there. Create an issue, possibly reference to the core issue.