Nodegraph inputs are not propagated to all connections

We are experimenting with `standard_surface_brick_procedural.mtlx` and changing its properties. It seems to me that the inputs in the nodegraph (for example `hue_variation`) are copied by value to each node that is connected to it, and then is only one of those nodes stored as a path for the input variable. Here is a simple repro to show what I mean:

```xml

```
When opening this in MaterialXView and changing `in_color` in Property Editor, only one of the inputs to the split node is updated:

Version tested: MaterialX main branch as of today (`a78d13f6`)

Start with the supplied XML repro in MaterialXView and change in_color in the Property Editor. Trace how the nodegraph interface input reaches both splitlr connections, then verify the issue is resolved when changing in_color updates both valuel and valuer.

Written by the indexing model from the issue text.