Point instancers with prototypes containing animated xform on instanceable prims don't update

[marktucker]

Description of Issue

When the prototype of a point instancer contains an instanceable reference with a time-varying xform on it, the xforms of the instances do not seem to be recognized as time varying by hydra. The instances don't move when changing the current time.

Possibly related to #1311

Steps to Reproduce

1. Load spinning_cube_instanced.usda into usdview from the attached zip file:
   instanceupdates.zip
2. Scrub the current time. The cubes should all spin, but they don't.
3. Changing the prototype to use a non-instanceable reference makes the point instancer instances rotate as expected.
4. Moving the xform so that it is on /geo/protos/cube/cube instead of /geo/protos/cube also makes this file work as expected.

System Information (OS, Hardware)

Windows and Linux

Package Versions

Tested with USD 20.08 and 21.08

[jilliene]

Filed as internal issue #USD-6900

[spiffmon]

Adding in backpointer to possibly related issue from usd-interest for testing when we're able to look into this.