1) What happened
Universal shader: Detail mask 'understands' only transparent areas of its texture

2) How can we reproduce it using the example you attached
See attached project, or:
1. Assign to Detail Mask a greyscale texture that has (as well) some transparent area
---> See that secondary maps diffuse texture blacked-out only in areas, that match transparent area of Detail Mask.
Expected: Any grey tint of Detail Mask should affect Secondary Maps overlay