I use 2 techniques:
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I drag the disconnected parts onto the main part (it’s a good practice to fix the position of that main part first), so it sticks to it’s surface. This will (in most cases) get rid of tiny space between them.
But as Defaultio already mentioned, it’s best to sculpt a shape out of one, main part.
And btw, this isn’t exclusive to CSGv2. Switching to v1 can’t affect this issue.