I want to add a new hair option. Due to the problems that often arise between ImprovedUI, mod fixer and new patches, I'd rather just use the loose files method and replace an existing hair.

I've had no problem with replacing heads this way. I only need to put the GR2 file in the right directory such as:
c:\Program Files\Steam\steamapps\common\Baldurs Gate 3\Data\Generated\Public\Shared\Assets\Characters\_Anims\Tieflings\_Female\Resources\TIF_F_NKD_Head_A.GR2

Using the same method for hair has proven more problematic. For example using this path:
c:\Program Files\Steam\steamapps\common\Baldurs Gate 3\Data\Generated\Public\Shared\Assets\Characters\_Models\_Hair\Resources\Hair_HUM_F_Straight_Long_A_Spring.GR2
Results in the hair not appearing properly as seen in the image. Any ideas what I'm missing?

[Linked Image from i.imgur.com]