Detail map normals and material normals are combined wrong when using different UV channels

If we have a material using a normal texture in UV 0, then apply a baked normal map as detail in UV 1, then we get wrong normals as a result. If however the material normal and detail normal use the same UV channel, we get correct results. When toggling the material normal UV channel it looks like this:

In this example, the normal texture in the material is a solid color, flat normal (127, 127, 255), so it shouldn't affect the end result.

If we swap the textures so that the baked normal map is in the material, and the flat normal map is applied as a detail, we get the correct normals no matter what UV channel configuration. Here's a comparison of the different combinations (the top left is wrong, the rest is correct):

The example project can be found here:
https://playcanvas.com/project/1184651
Try toggling the material TestObj's normal UV channel. When set to UV 0, we get wrong normals. When set to UV 1, we get correct normals. The expected result is that there shouldn't be a visual change when using different UV channels.

While the example project may not look that wrong, it's very visible when working on real models:

Tested on both Firefox and Edge, and using the latest engine version as well as older ones.

Start with the linked PlayCanvas project and reproduce the issue on TestObj by toggling the material normal UV channel between UV 0 and UV 1. Trace the normal-combination path for different material and detail UV channels; done means the rendered normals remain visually unchanged when the channel is toggled, including the reported texture combinations.