Nested material function results in incorrect reference chain

Opening the Reference Viewer for a nested material function used indirectly through another material function shows that it is directly referenced by the material, rather than through the other material function. This makes it hard to see which graphs the material function is actually directly placed in.

1. Create a material function mat_func1 that outputs a constant (or whatever)
2. Create a material function mat_func2 that uses mat_func1 in some way
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3. Create a material that uses mat_func2 in some way
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4. Opening the Reference Viewer for the nested material function shows that it is directly referenced by the material, rather than through the other material function
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