When Google announced its Pixel 4 and Pixel 4 XL handsets earlier this month, it made some curious choices, such as the large top bezel, the lack of 4K 60fps video capture, the omission of an ultra-wide sensor, and the relatively small batteries. One of the key features of the devices is the 90Hz OLED display, something that we've seen from few other OEMs, but as it turns out, Google made another odd choice with this one: it only works if the brightness is over 75%.
Originally spotted by a user on Reddit, it turns out that this is indeed correct. If your brightness is below 75%, the refresh rate drops to 60Hz.
It's unclear why Google decided that it should be this way, but there's good news. You can actually force the screen to be 90Hz all the time, but you'll need to do it through developer settings and it will likely affect the already sub-par battery life. To turn on developer options, you can go to Settings -> System -> About phone, and tap the build number seven times. Find the option to force 90Hz, and turn it on.