Merging Actor messes up Static Mess settings. Bug?

I have a number of houses which I edited from elements in an empty scene and then create with “Merge Actor”. When these meshes are placed in a scene I get warnings that “ConvexElem[0] has negative scale”

A closer inspection shows that the static mesh settings show an array of convex elements, but none of these elements contains any data. Clicking on the yellow reset arrow has no effect. The array can not be deleted. I have to copy the box element array to the clipboard, reset the whole Primitive sections, then paste the box elements again

This sounds like a bug to me, but I want to ask first if I overlook something or not?