Force a Mesh to update its acceleration structure

When i join two meshes a and b with:
it does not update its internal acceleration structure, so a.ClosestPoint(..) only sees the original a mesh part…
My workaround is to create one double vertex, run Weld() and then the structure updates.
Any clean way to do this?

Still on V5 as V6 meshes are very broken: RH-46728 - hope someone fixes this soon…

It seems to work as expected on my system (Rhino 5.14.522.8390, 05/22/2017), unless I’m not understanding the issue: (8.0 KB)

I now tried to isolate and reproduce the problem - and didn’t succeed…
Guess i have a bug somewhere else… sorry for the spam!