some point your server lost power. The info in the stats collector gets reset on recovery.
Haven't pondered the vacuum side of the equation but ISTM if the auto-analyze daemon is confronted with total amnesia that it should go ahead and schedule an analyze on the entire database. ANALYZE is relatively inexpensive.
And maybe learn some techniques to compensate. Like copying the most recent snapshot of its input data to some place that survives a crash-restart.
Also:
(formula) "is compared to the total number of tuples inserted, updated, or deleted since the last ANALYZE."
