OP here, back from holidays....
Thanks for all your responses.
Turns out Apple's Thunderbolt 3 to Thunderbolt 2 adapter strips out the DisplayPort signal :-(
I purchased a USB-C to DisplayPort cable (not an adapter) and it works fine now at the full UHD resolution at 60hz.
BTW, the NEC PA322UHD is phenomenal. Maybe not as good as an Eizo, but for mere mortals it is the best thing going.
And can't complain about the 2016 MacBookPro either, amazing screen for a laptop, and the 2TB internal SSD (with something like 1,100Mbs read/write speed) sure is handy.