As of 9.0, it looks like addons aren’t supposed to replace IsContainerItemAnUpgrade anymore—if you do, you’ll get taint errors when swapping weapons via the character sheet while in combat. The Pawn addon unfortunately needs to replace that function to be able to enhance the criteria under which the green upgrade arrows in bags appear.
Any chance that’s an oversight that can be fixed? To work around this, Pawn has to replace the bag code that calls IsContainerItemAnUpgrade, which has the nasty side effect of Pawn no longer being able to integrate with custom bag addons without Pawn-specific code in the bag addon.
(To reproduce the problem, use /script IsContainerItemAnUpgrade = function() return false end, enter combat with anything, and then hold down Alt while you hover over your weapon on the character sheet, and then choose a different weapon a couple of times until you get a taint error and your weapon slot gets locked.)