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)
Results: Compile Error
Expected: Successful Compile of Cube_AnimBP
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-62613 in the post.