The nbt tag from the festive affix is added to items in the players inventory, but not removed when keepInventory is enabled, as they are never dropped.

This prevents items kept through deaths from being able to be stacked with regular items of the same type.

Tested with Apotheosis-1.18.2-5.7.7, Placebo-1.18.2-6.6.6, and Forge-1.18.2-40.2.1,

It hasn't been tested with the 1.19 version but I'd assume the bug also exists there, as the code is mostly the same between the versions.