The chicken dinner banner doesn't seem to be working... Unless all banners are active the moment you get them and it's not the current one equipped in your room that matter... If that is the case I may have missed something
I tried it on the alchemist when you're chasing those Firefly's (15 points) and without it also got (15 points), I tried with the training at the tree dumbbells... And where it just shows Marla also only offered (15 points)
So it's not "Doubling" anything (at least in the message to players) about to check to see if it really is behind the scenes... And according to stats (Before using chicken dinner) completed task and checked stats, and after Chicken Diner is Equip (During using chicken dinner) completed task and compared this stat to the non banner version with the same result.