The bug

Minecraft generates mipmapped versions of the armor trims atlas file. This is despite the fact that armor trims are unaffected by mipmapping.

Other things with atlases which are unaffected by mipmapping, such as particles, paintings, entity effect icons, GUI textures and map icons, do not have mipmapped versions of their atlases generated.

Steps to Reproduce:

1. Join a world
2. Press F3 + S to generate debug textures
3. Click the link posted in chat to go to your screenshots/debug folder
4. Find minecraft_textures_atlas_armor_trims.png_2/3/4

How to fix

Do not generate mipmapped versions of this atlas.

Further notes

Is it at all possible to configure mipmap levels in atlas files? I can't see any parameters implying this.