During runtime, a data only blueprint's class defaults tab becomes greyed out and uneditable during runtime. The values for specific instances can still be edited via the detail's pane.
1. Open Editor (any project)
2. Create new blueprint
3. In blueprint, create 4 vector variables, set to editable, expose on spawn, and show 3D widget.
4. Compile blueprint
5. Create a child based on new blueprint (RMB>Create child based on this blueprint)
6. Place a copy of the child in level
7. Open child blueprint
8. Press PIE.
Class Defaults page grays out and cannot be altered.
Defaults page remains open and can be altered, but all changes revert when PIE is closed. This would follow the same principles as changing values in an event graph or component viewport.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-19769 in the post.