I would not panic just yet, but I would not ignore it either.
The fact that your original 24-inch M1 iMac has briefly flickered several times over the past few days, including during startup and shutdown, is worth looking into.
A flicker lasting just a second or two does not necessarily mean the display is failing. It could be a temporary software or graphics issue, but given the reports of display problems affecting some original M1 iMacs, a hardware issue can not be ruled out.
Since the iMac was introduced in 2021, some owners have reported display deterioration, including horizontal lines, dark bands and other visual abnormalities.
These reports have been covered by Tom's Hardware and TechSpot.
Your symptoms are not exactly the same as the persistent lines and dark patches described in many of those reports. A brief flash across the entire screen could have a different cause. Still, I would not dismiss the possibility of a developing display problem simply because the flickering disappears so quickly.
Could macOS 27.0.1 be responsible? Possibly. Software or graphics-related glitches can cause temporary display abnormalities, but being fully updated doesn't rule out a hardware fault. Since you've noticed the flickering during normal use, startup and shutdown, it's too early to blame the operating system.
I would start by making sure you have a current backup of your important files. Then keep an eye on the screen and note whether the entire display flashes brighter and darker or whether the flickering is concentrated in one area. Horizontal lines, dark patches or increasingly frequent or prolonged flickering would make a hardware problem more concerning.
You could also try starting in Safe Mode using Apple's official instructions. If the problem continues there, it raises the possibility of a hardware or lower-level system issue, although it isn't conclusive.
It's also worth running Apple Diagnostics. On an Apple silicon Mac, shut it down, press and hold the power button until the startup options appear, then press and hold Command-D. Follow the instructions and note any reference codes. Keep in mind that an intermittent display fault may not show up during the test.
Apple's official M1 iMac repair manual includes troubleshooting guidance for display and image issues. If the flickering happens again, I'd contact Apple Support or an Apple-authorised repair provider and ask for the display to be assessed. Explain that the screen briefly flashes brighter and darker at different times, including startup and shutdown.
It's worth mentioning the reports from other M1 iMac owners and asking whether any repair assistance is available. However, those reports don't establish that every affected iMac has the same defect, and Apple hasn't confirmed a widespread display problem or a dedicated repair programme in the sources cited above.
My take: There's no reason to assume the worst based on a one-second flicker, but because it's happened several times, I'd keep a close eye on it rather than dismissing it as a macOS glitch. Back up your data and arrange an inspection if it continues or becomes more pronounced.
One question that might help narrow things down: when the flicker occurs, does the entire screen flash brighter and darker, or does it seem to come from a particular area, such as the bottom of the display?
