I believe there are many others out there unfamiliar with this novel technology who may not realize the shortcomings with both normal and vivid HDR modes, but who also feel something isn't quite right with their only 2 options. Tft central noticed it as well, and I've posted a link below. I don't understand why Dell would manufacture and advertise a premium HDR UHD alliance certified display with FALD backlight, 1000 peak nits, and wide color gamut and then ship it with seemingly minor programming oversights that keep users from experiencing this powerful and expensive hardware in a practical way. I would argue that it is a systemic fault, and essentially makes HDR mode unusable because it's an overall picture quality downgrade from SDR (UHD alliance certified HDR is the main driving point of this monitor).
To fix the issue all Dell would need to do is add an HDR brightness setting in a firmware update (every HDR TV I've used has one of these, I honestly don't know if it's a different situation with monitors but I don't imagine it would be).
The justification would be to do it for the people out there using their UP2718Q's for HDR who aren't getting the experience they hoped for and paid for. We know of 3, I'm sure there's more, and I believe there are people using it who don't quite understand the issue but notice it as well (or just think that's how HDR is supposed to be). It's your flagship product, fix it, what else are you going to do?
"We found some somewhat mixed results with the HDR option though from the PS4 console. In the 'vivid' HDR mode there was excessive blooming, to the point where it looked really bad. You got massive bright blooms around bright areas, for instance in the loading screen where you have a small spinning coin on a black background, and a small white progress % shown. Those really highlighted the excessive blooming and bright halos. It looks almost like really bad IPS glow or backlight bleed, but it was a result of the local dimming backlight. It was not as obvious in normal gaming where you have far more varied content, but the vivid mode seemed to produce too much of this glow. We did find the 'normal' HDR mode much better though thankfully, and the blooming was only slight on the aforementioned loading screen. Again in actual gaming you couldn't really detect any major problems. This mode did seem to be darker overall though, so perhaps there is some limitation of the peak luminance range going on here. The user manual doesn't mention anything, but there is a marked difference in how bright the bright areas appear between vivid and normal. Although as we said, the vivid mode then results in a massive amount of blooming. We have heard some reports that there is some noticeable lag from the FALD as well, where it seems to take a noticeable amount of time to change a zone on the backlight as content changes. E.g. a bright object moving across a dark background will show a lag to the dimming zones and therefore a bloom trail behind it. We will try to produce some further tests and update this part of the review if we can and provide some further thoughts (updated tests in a moment below)."