Couldn't recreate, but also not sure how I'm supposed to recreate. Could you give some more precise steps?

1. Create 2 bones: A and B
2. Add a constraint on B
3. Split properties editor: one editor for bone,the other one for bone constraints
4. Select bone B and pin the constraints editor
5. Select bone A. Constraint disappears.
6. Pin bone editor to show bone A
7. Select bone B again. Constraint reappears, but bone settings now show bone B again, not bone A

Here's the original file where I noticed the problem (it's equivalent to step 5 above - A = BBone, B = green/bone with constraint)

Afaik properties pinning never worked for bones? It’s ID feature only (cf. sbuts->pinid = buttons_context_id_path(C);, and buttons_context_id_path() in buttons_context.c of buttons space editor), and bones are no ID ;)

So sounds more like TODO than actual bug to me, though can see how useful this feature would be, and probably not too hard to implement even.

Code works as intended only with IDs, which bones aren't. Changing to ToDo.