Its basically impossible to select no overwrite for any terrain piece in the recent NLEditor. This option is essential for grouping because drawing earlier would only make the piece get erased by an eraser piece, while no overwrite can put it behind the piece without erasure.
Why is this gone? and where is the previous version of the Editor so I can get this function back?
Fixed in commit b0732e0.
Thanks for reporting, I'll get a hotfix out soon. In the meantime, use the No Overwrite hotkey (N) to set pieces to No Overwrite. It will still work, it's only the checkbox availability itself that wasn't being set properly.