At the end of April, Acer showed a number of new monitors, including the Predator X27 with G-Sync, HDR and a 4K resolution at 144Hz.
At that time, Nvidia promised to sell G-Sync HDR screens in the second quarter of 2017, but it appears that (at least for Acer) did not succeed.
In fact, Acer has announced via Twitter that the Predator X27 has been postponed to the first quarter of 2018. A reason is not given, but perhaps AU Optronics (the panel manufacturer) might have encountered problems in production.
The goal is to support HDR10 with this G-Sync HDR display, reducing backlight in 384 zones for better contrast. Certainly, we do not actually see that on the screen, so the core of the problem might be there. The real cause, howeve
r, remains pure speculation.