The issue:
Regardless of dominant hand, some items are held sideways when in the left hand. While only graphical, it looks very odd when having said items in your left hand, while playing left handed, & even more odd when having the affected items in both hands.

The reason:
While this doesn't seem to visibly affect every item, every item still gets treated the same in the left hand; this means the issue lies in the way items are rotated in the left hand, and the effect only becomes visible with certain textures.

In my testing, I have found a good majority of items with diagonal textures (or in other words has a texture that goes from corner to corner) end up encountering this rotating issue, but this doesn't include all of these diagonal textures, which also includes tools. The reason for this is the parent file that is being used!

The affected items use "generated.json", while tools and other items that are not affected use "handheld.json". It is however not recommended to use "handheld.json" as the parent for the affected items, as it doesn't just change the left hand position, but also how the items are held in 3rd person too.

For example, the bucket has a texture that goes top to bottom, and is unaffected. As seen here:

But shears, just like all tools, has a texture that goes corner to corner, and are turned sideways when in the left hand. As seen here:

An easy fix:
The easiest way to fix this issue, is to use the same method that fixes the tools, and add a "firstperson_lefthand" case into "generated.json", specifically:

"firstperson_lefthand": { "rotation": [ 0, 90, -25 ],       "translation": [ 1.13, 3.2, 1.13],       "scale": [ 0.68, 0.68, 0.68 ] }

for all items except the Clock and Compass, as they must be rotated in order to work correctly. Not only does this fix the issue, but it also doesn't negatively affect items that weren't effected beforehand!

An example with this fix in place is here:

And here's the bucket again, but now with the fix in place:

List of visibly affected items:
Shears, All Arrows, All ingots, Brick, Netherite Scrap, Lapis Lazuli, Book, Enchanted Book, Book and Quill, Feather, Paper, Sugar Cane, Wheat, Bread, Sweet Berries, Rotten Flesh, All Fish (Raw or Cooked), Carrots, Porkchop (Raw or Cooked), Beef (Raw or Cooked), Kelp, Dried Kelp, White Dye, Yellow Dye, Green Dye, Banner Pattern, and more.

Final notes:
Despite being diagonal, Sticks, Bones, Blaze Rods, & Bamboo aren't affected, since they use the parent file "handheld.json" and not "generated.json"

I have supplied a resource pack made by myself as a fix for this issue in the attachments, or alternatively here!: Left Hand Fix.zip