Nakai and most likely other factions are losing vassal income after a save is reloaded in a campaign. I recently tested it out, and the effect occurred today during a brand new 5.1 Nakai campaign. Vassal income appears to reset to 0 no matter how much it was after a save is loaded back up. I did see the forums that said this was only for old campaigns pre-5.1 patch, and that's why I specifically tested this out with a new campaign.