Vertex Painting changes collision complexity if the asset is saved while vertex painting. The collision change is set to "Use Complex Collision as Simple". If this is intended functionality there should be an indicator or warning message that the collision complexity will change while vertex painting.
Regression (Yes) issue (does not) occur in 4.10.4
Results
Collision complexity is set to "Use Complex Collision as Simple"
Expected
Either Collision Complexity is set to default or an indicator appears while painting that the collision complexity of the asset will change if vertex painting is used.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-30661 in the post.