Since upGrading to macOS 27.0, your Apple Silicon Mac has been turning off the display or going to sleep during active video playback.
You report that this did not occur before the upGrade.
First, determine whether only the display turns off or whether the Mac actually goes to sleep.
If the screen goes dark while audio continues, that is display sleep; if playback stops and the Mac enters sleep, that is system sleep.
Check Apple menu >> System Settings >> Lock Screen and review the display sleep settings.
Also check Apple menu >> System Settings >> Energy for sleep-related settings.
Apple provides information here: Set sleep and wake settings for your Mac and Change Energy settings on a Mac desktop computer.
While a video is actively playing, open Activity Monitor >> Energy and check the Preventing Sleep column for the application playing the video.
This can show whether that application is currently preventing sleep.
Then test the same video in Safari and another browser. Note whether the display behaviour and the Preventing Sleep status differ between applications.
The fact that caffeinate -d prevents the display from turning off is useful diagnostic information, but it does not establish the underlying cause.
If active video playback previously kept the display awake on your Mac, you would not ordinarily expect to need a Terminal command simply to watch an actively playing video.
If the display continues to turn off during active playback after these checks, the behaviour can be documented as a possible change or problem following the macOS 27.0 upGrade or as an application-specific issue, depending on the test results.
If the problem persists, report it to Apple Feedback Assistant. Include your Mac model, macOS 27.0 build number, application used for playback, whether the display or the entire Mac sleeps, and what Activity Monitor shows while the video is playing.
caffeinate -d can be used as a temporary workaround, but it should not be necessary merely to watch an actively playing video.
