Why can't Preview Export 16-bit HEIC images?
Apple doesn't appear to support 16-bit HEIC.
I can't find anything official on this, but I can find a whole lot of user complaints bemoaning it.
https://discussions.apple.com/thread/251072024
I'd be seriously tempted to try Nikon's own ViewNX-i [freeware] to do your conversions. Though as yet it doesn't support HEIF at all, it is far better at interpreting Nikon RAW files than any other app. Overall you will probably lose less information going NRW &gt; ViewNX-i &gt; PNG than you would via Preview, or even Photoshop.
BTW, NRW isn't 16-bit, it's 12-bit. Nikon's higher spec RAW file NEF is 12 or 14-bit.