Modifying the purpose of a scenegraph instance doesn't trigger a sync of the instance prototype

[williamkrick]

Description of Issue

When the purpose of a scenegraph instanced prim is modified there is no call to sync for that prim.

Steps to Reproduce

1. Unzip and open the attached file instanceAndNot.zip in USD View. In the scene the 5 cubes (1 hidden) are scenegraph instances and the one capsule is not an instance.
2. View -> Display Purposes -> Uncheck proxy
3. Select Capsule1 and choose the purpose property in the property window. It's value should be "default".
4. In the python interpreter modify Capsule1's purpose to be proxy: usdviewApi.property.Set('proxy')
5. Capsule1 correctly disappears.
6. Select ball_01 and choose the purpose property.
7. Modify the purpose to be proxy in the same way: usdviewApi.property.Set('proxy')
8. ball_01 does not disappear.

When I try something similar in MayaUSD with the VP2RenderDelegate I get a similar result, the instance whose purpose changes does not disappear. Debugging in MayaUSD I found that there is no sync call for the instance prototype and so no opportunity for VP2RenderDelegate to change how it draws. When I force a sync (ogs -reset in Maya) and query the instancer I do find that
the instancer only has 3 instances in it rather than 4, and I am able to draw correctly in that case.

System Information (OS, Hardware)

Windows 10

Package Versions

USD v21.05

Build Flags

[jilliene]

Filed as internal issue #USD-6686