Hi,
This code is very experimental at this stage, but it’s definitely something we’re looking to add. I would have expected the COM to be correctly recalculated as the call to FBodyInstance::Weld calls UpdateMassProperties()
From my initial tests the objects seemed to show the right mass distribution, but I have not tested this with applying torque like you say.
I’ve been working on other features for the moment so I haven’t looked at this code very recently, but I would expect that when we officially release this it will have to fix the issue you’re describing. Having said that, if you spend time investigating this I’d certainly appreciate it!