Macbook screen brightness keeps pulsing with strange lines

Based on your description (especially the pulsing brightness and horizontal lines), it’s likely either a hardware-related fault with the display or a logic board/display connector issue. Since you've already tried the usual suspects — turning off Auto-Brightness, True Tone, and battery dimming — we can probably rule out software-driven adaptive brightness.

Here’s somet things you can try before heading to Apple support:

Reset the SMC (System Management Controller)

• Although modern Apple Silicon Macs like the M3 don’t technically have a resettable SMC like Intel Macs did, you can simulate the effect:
  • Shut down your Mac completely.
  • Wait 30 seconds.
  • Turn it back on.

Boot into Safe Mode

1. Turn off your Mac, then press and hold the Power button until “Loading startup options” appears.
2. Choose your startup disk, then hold Shift and click “Continue in Safe Mode.”
3. Use your Mac for a few minutes and check if the issue persists. If it doesn’t happen in Safe Mode, you might be looking at a third-party software or extension issue.

Check for macOS Updates

• It’s always worth checking System Settings > General > Software Update. Apple’s been known to patch graphics glitches on newer hardware with post-launch updates. The most current version is now: 15.4.1

External Display Test

• If you have a USB-C to HDMI or DisplayPort cable, try plugging into an external monitor. If the external display is fine while your built-in screen glitches, it’s very likely a display panel issue (or its connection to the board). If both show issues, then the GPU/logic board might be at fault.

If none of that resolves it and you’re still seeing pulsing brightness and artifacts, I’d definitely recommend booking a Genius Bar or authorized service appointment. The fact that it started out of nowhere and includes horizontal lines hints at either a failing display panel, a loose or defective display cable, or worse, an issue on the logic board itself — all of which shoud be covered under Apple’s 1-year warranty (or AppleCare+, if you’ve got that).