The bug

Most GUI sprites either follow or make an attempt to follow what is specified inside of the associated .mcmeta file with respect to the "scaling" parameter. However, for some reason, some of the icons used for certain empty inventory slots ignore them entirely.

How to reproduce

A resource pack to test this is attached.

The changes made are as follows:

• helmet slot uses "nine_slice"
• chestplate slot uses "tile"
• loom dye slot uses "nine_slice"
• loom banner slot uses "tile"

Expected results

Both the helmet slot and loom dye slot would appear as a pale colored square with a dark border. The chestplate slot and loom banner slots would appear as a thick grid with small pale squares.

Actual results

We see the correct behaviour for the loom slots. However, for the helmet and chestplate slots, we incorrectly get a square with a thick red border.

How to fix

This could be fixed easily by moving the affected slot sprites to the gui/sprites directory; they probably shouldn't be in the items directory as they're not textures used for items, and are exclusively used for drawing sprites to the user interface. By being stitched to the gui atlas, proper scaling should just work.

A suggested path would be textures/gui/sprites/container/empty_slot. The following files would need to be moved to there, and most likely renamed accordingly:
textures/item/empty_armor_slot_boots
textures/item/empty_armor_slot_chestplate
textures/item/empty_armor_slot_helmet
textures/item/empty_armor_slot_leggings
textures/item/empty_armor_slot_shield
textures/item/empty_slot_amethyst_shard
textures/item/empty_slot_axe
textures/item/empty_slot_diamond
textures/item/empty_slot_emerald
textures/item/empty_slot_hoe
textures/item/empty_slot_ingot
textures/item/empty_slot_lapis_lazuli
textures/item/empty_slot_pickaxe
textures/item/empty_slot_quartz
textures/item/empty_slot_redstone_dust
textures/item/empty_slot_shovel
textures/item/empty_slot_smithing_template_armor_trim
textures/item/empty_slot_smithing_template_netherite_upgrade
textures/item/empty_slot_sword

The following could also be moved:
textures/gui/sprites/container/loom/banner_slot
textures/gui/sprites/container/loom/dye_slot
textures/gui/sprites/container/loom/pattern_slot