When a behavior tree contains a Run Behavior node, opening the behavior tree marks the asset as "modified" and users would get asked by the editor if they want to save it even when there has been no change.
This does not happen with Run Behavior Dynamic node.
1. Create a behavior tree
2. Add a sequence then a Run Behavior node
3. Close the behavior tree editor and save the asset
4. Open the behavior tree again - there will be "*" at the end of the tree name showing the asset has been modified
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-43430 in the post.