This is an artefact which occasionally (~once in several hours) happens on displays that use AU Optronics M320QAN01 32" 4K panel and similar 27" 4K from AUO.
You can see it in real time around ~3s from start.
Last known list - Acer XB321HK, Acer XB271HK, BenQ BL3201PT, QNIX UHD3216R, CrossOver 324K, HP Spectre 32 Studio (may be more)
This was recorded by iPad Air on Acer XB321HK connected via HDMI interface in 1080p 60Hz res - however issue happens in all other resolutions and refresh rates I've (and other people) tried.
As with their other problematic hardware, Acer was notoriously bad in handling this issue. For example, during whole year since it was first discovered they've claimed that they "can't reproduce" it. Only recently one of their forum reps admitted the issue was reproduced, but no definite resolution and/or fix was promised.
Update: after 2 months after acknowledging they reproduced the problem all that Acer did *they've deleted most of the thread with people complaints* (including their acknowledgement of the issue). And since that, they went dead quiet and never replied again. So I assume they will pretend problem does not exist - after all, its too expensive to fix monitors they've already got the money for...
Update 2 (15 March 2018) - Acer have finally replied to thread on their forum. They've claimed that they have identified the fix, and it took so much time because diagnosing and making sure that problem is fixed was difficult due to randomness and rarity of glitching. They stated that information was passed to all regional services who should be able to perform required repair work. Also they've claimed that deleting several months worth of complaints was not intentional and just a glitch due to forum provider move.
Update 3 (June 2018). One customer decided to take on Acer's word that problem is diagnosed and they should send their monitor for repairs. At the end, they have wasted ~3 months (!) while having Acer "trying to repair" their monitor, receiving it back *several times*, and every time issue was not fixed. Best Acer could achieve is eliminate glitches if G-Sync completely turned off - but this kind of kills whole point of having G-Sync monitor. Acer also tried to give them "replacement" monitors two (!) times, both replacements still had the glitch. At the end Acer "generously" offered them refund of $400 (originally they've paid ~$1000).
Last Update (August 2018) - Another "solution" Acer-style - they've closed both forum threads (both for xb321hk and xb271hk), posting usual "send your monitor to nearest service center to replace your panel" and marking it as "accepted solution" by themselves. Note how on same thread just previous posts from very upset customer state how they've tried this "solution" and it didn't help them to get artefact fixed - obviously this was totally ignored by Acer reps (and I expect will be deleted after closed threads will fade to obscurity). Can't make potential future customers feel anxious about potential issues with their 1K$ monitor, I guess.
More details in Acer own forum thread (part deleted, and closed now) -
https://community.acer.com/en/discuss...