Seems that only the Add Asset Class search option works properly.
Tested in 4.19.2 (CL - 4033788), 4.20.1 (CL - 4233996), and 4.21 (CL - 4280116)
Expected: Both of these search options should produce the PrimaryAssetLabel_BP Blueprint in its search results
Results: Only the Add Asset Class button only works as intended
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-62872 in the post.