Am I mistaken, or was this supposed to be fixed. I had just gotten the acheivement for capturing 20 souls, and the next one I shot out of the air. It crashed down, I got the bones, but it didn't dissolve. What's the deal?

Was it a dragon you visited before? There are respawning dragons at locations (Like word walls) that do not give souls to prevent farming for souls.

It didn't dissolve? Bug.

Or respawning dragons ^

This is most likely your case. I have encountered this before.

You're supposed to be able to get souls from all dragons you kill, including the respawning dragons. It is a known bug. Preventing 'farming' in this instance doesn't make any sense, as you're still killing a dragon (and you're not playing an MMO).