It’s not in Blender where the deleted bones are reappearing but Unreal Engine. The only reason I can think of why happening is because the main character model & all of its animations are being imported into Unreal Engine from two different Blend files. The Blend file with the main model is the one that I deleted the “modifier” bones from, while the other one containing all of my character’s animation maintained those bones because I needed the Inverse Kinematics bone restraint to animate my character. Instead, I tried selecting all necessary bones while leaving out the “modifier” bones that were not compatible with Unreal Engine & turned on Selected Objects in the Export FBX options. I guess its possible that the Selected Objects option didn’t work as I intended and instead imported the entire armature rather than just the bones I selected.
However, while I would prefer to get rid of the incompatible bones that serve no purpose in Unreal Engine, none of explains why the “grip” bones I added to my model for the purpose of attaching the weapon to isn’t working properly, making it my primary concern.
I have no idea why the “grip” bones aren’t working, the only thing I’ve noticed is that the two bones aren’t listed in the Dope Sheet Summary for the vast majority of the animations I’ve created so far, but I’m not sure whether that is something to be concerned about or not.