The developers of the Fix Mutagen Stacking in New Game Plus have released a mod to fix the issue of the mutagens not stacking in the game. This mod allows the players to import and use more than one mutagen at the same time. It also allows them to keep the original stats of each mutagen.

In the first place, this change was made to the game by patching the files. The developers forgot to make the same changes in the NG+ file, which caused the problem of the players not being able to import more than one mutagen. In the meantime, the developers have released a patch to fix the problem.

The problem with importing multiple mutagens at the same time is that the player needs to change the item from the inventory. This change is made by editing the item XML, which is a conflict with another mod that modifies the same element.