Hi! I just find out that my kitchen design application is broken after the latest Babylon JS v6.21.0 and v6.21.1 update - with version < 6.21.x it was working fine.
I think that problem is BABYLON.Mesh.MergeMeshes function. Here are some screenshots before and after the BJS updates from 6.20.x to 6.21.x. Kitchen modules consist from couple of different parts and the frame is merged together from these parts with parameters MergeMeshes(parts, true, false). When i now use those same parameters my kitchen modules merged frame is disappearing.
I don’t see what could explain this problem, as there are only a few changes between version 6.20.2 and version 6.21.0, and all these changes are either related to the node geometry editor, or to the lens / SSAO / SSAO2 rendering pipelines:
So, on the face of it, nothing that would have an impact on MergeMeshes.
Do you have a reproduction somewhere for us to take a look at?
Okey I created now the scene to playground and got the same problem reproduced.
If you switch BJS to v5.71.1 it works as expected and with LATEST (it’s now v6.21.1) some parts disappear. In fact those parts that are disappearing are the others than those that are merged…!
I’ll try to figure out how could I reproduct this with playground or with some other way - I’ll be back! 
no bug for more than 24hours ;D