Not sure if it's caused by optifine, but for some reason the couplings mod icon replaces the icon of the default texture pack

It doesn't do this if you move it to assets/pack.png instead of just pack.png.

This is a problem with how Fabric's API handles resource packs.

Mod jars are automatically loaded as resource/data packs. Just because they are not visible in the GUI does not mean they do not exist. If you queried my resource/data pack via the resource manager, you would be exposed to my resource/data pack icon.

The issue is on Fabric's end for using a file from an arbitrary jar for the vanilla resource pack icon. Vanilla does not have this issue with zip/folder resource packs, it is an issue exclusive to Fabric's resource loading. Mods shouldn't be expected to work around this bug.

It is my resource/data pack icon, it is supposed to be where it is.
Fabric using it for an unrelated resource pack is not my mod's problem.

But it's not a resource/data pack icon, it's a mod icon. You don't have a visible resource/data pack for the icon to apply to; it's hidden in the resource pack view.