Detail mask only applies UV1 even when set to UV2

- *Production edit: Related to https://github.com/godotengine/godot/issues/54644.*

### Tested versions

v4.3.stable.steam [77dcf97d8]

### System information

Windows 10

### Issue description

Just trying to get a hang of material importing in Godot and I think I found a really annoying bug. I was making a road texture with the road lines as a detail texure on top. The asphalt and the roadlines have 2 seperate UVs that both imported properly

I used the detail section of the material to put the roadlines on top and set it to UV2. The roadlines need a mask to mask out just the roadlines.

Everything works fine without the mask:

But when I add the (obviously needed) mask to the detail section it reverts to UV1 even though its still set to UV2

### Steps to reproduce

Take a mesh with two UV maps.

enable Detail in material, Set Detail to UV2, add Detail Mask

### Minimal reproduction project (MRP)

[detailmask_bugreport.zip](https://github.com/user-attachments/files/17440750/detailmask_bugreport.zip)

all you have to do is go into the surface 0 material on the included model and under Detail/Mask add the Roadlines_mask.png

before you add that mask you can see UV2 is working properly. After you add the mask you can see it's still using UV1