Using a Sub Anim Instance causes a "Sub Instance Loop" Compile Error

Using a Sub Anim Instance causes a sub instance loop error when referencing another Animation Blueprint. This is a regression from 4.19.2 (CL-4033788) as it complied without error.

This issue was reported and tested in 4.20.1 (CL-4233996) and reproduced in Main 4.21 (CL-4258917)

1. Download and open the attached project
2. Open Cube_SubAnim to look at the setup
3. Open Cube_AnimBP and try to compile it

Results: Compile Error

Expected: Successful Compile of Cube_AnimBP