Starting with VS2019 16.2.0 Preview 3.0, the C++ 'Extend multi-line comments' feature is adding an extra newline after pressing Enter, bringing the caret into a new empty line just below the auto-extended new comment line.
Apparently the issue is only present when I have the "IncrediBuild Build Acceleration" (version 184.108.40.206) extension enabled. (I don't think that's the same version as the one available for download on the VS Marketplace – I have mine installed through IncrediBuild's own installer.)
As far as I know, the IncrediBuild extension does nothing with the editor at all. I certainly don't know what it should be doing with the editor.
So, thanks for solving my immediate problem, though the actual issue remains a strange one...
I've received a response from IncrediBuild support: "we are working on the fix. The problem is with the IB add-in so one of the Solutions is to revert the add-in version back to 220.127.116.11 (no need to downgrade IB)."