Skeletal mesh bones cannot be selected in the viewport of the IK Retargeter. They can still be selected via the bone name hierarchy.
Reproduces in the following:
//UE5/Release-5.1 CL 2390901
//UE5/Release-5.2 CL 24887336
1. Create new Third Person Template in 5.1 or 5.2.
2. Add a new IK Retargeter in the content browser.
3. Set SKM_Manny as the source, and SK_Mannequin as the target.
4. Select Edit Mode on the left side of the IK Retargeter window
Skeletal mesh bones cannot be selected in the main viewport
The bones can be selected in the main viewport when edit mode is active
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-184901 in the post.