The Base Color slot should not be connected to anything when importing the glb file.

That isn't possible, the base color and alpha map are always packed together in glTF. If there is an alpha map, there must also be a base color map.

So looking at your "After Export" image, it looks like it will be correct as long as the exporter has made the RGB channels of the _COLOR texture completely white. Has it?

I confirm the analysis on Scurest

The Base Color slot should not be connected to anything when importing the glb file.

That isn't possible, the base color and alpha map are always packed together in glTF. If there is an alpha map, there must also be a base color map.

So looking at your "After Export" image, it looks like it will be correct as long as the exporter has made the RGB channels of the _COLOR texture completely white. Has it?

Sorry, maybe I expressed myself a bit wrong and used the wrong term, it is not an alpha map in this case (white/black) but the alpha channel of an albedo/diffuse texture. But as you can see, the color output of the Abledo/Diffuse map is not connected to the base color input of the Principles BSDF node before the export. If GLTF always packs base color and alpha together into one texture, then in this case the color output should not contain any color information and should be white as you say, right? Unfortunately, this is not the case here and the original color information remains in the texture. After reading your comment, I assume that alpha channels are not automatically converted to alpha maps?

Unfortunately, this is not the case here and the original color information remains in the texture.

Can confirm. Here is a .blend test case.

AlphaMapTest.zip