I have a texture pack that makes the GUI of Minecraft darker. But when opening the backpack it uses some hidden texture that only matches the default Minecraft GUI, this could've been a bug report but I'm more-so requesting for it to use something in the packedup:textures/ directory

The textures used for the background and slots come from my core lib and are shared between many of my mods.
https://github.com/SuperMartijn642/SuperMartijn642sCoreLib/tree/1.16.2/src/main/resources/assets/supermartijn642corelib/textures/gui
These should be the textures you're looking for, specifically the background.png and slot.png files.

Additional, there are some textures in Packed Up for the inside corners between the backpack and player sections of the gui.
https://github.com/SuperMartijn642/PackedUp/blob/1.15/src/main/resources/assets/packedup/textures/corners.png

Thank you so much! I didn't even think about checking the core, that's my fault. Thank you again