MacBook Air M1 Dock with autohide not appearing on extended display

Your configuration should support a Dock on the external display. 

Apple states that when Displays have separate Spaces is enabled, the Dock should be available on all displays. (Apple Support)

First, open System Settings >> Displays and confirm that the external monitor is shown as a separate extended display and is positioned above the MacBook's built-in display, as intended. 

Apple documents that display arrangement is configured from System Settings > > Displays. (Apple Support)

Then open System Settings >> Desktop & Dock and confirm Position on screen: Left and Automatically hide and show the Dock are enabled. 

Move the pointer onto the external display and directly against its left edge.

If the Dock does not appear, temporarily turn Automatically hide and show the Dock off. 

If the Dock then appears on the external display, that would indicate that the problem is associated with the Dock's automatic hiding behaviour.

If it still does not appear, restart the Dock with Terminal:  killall Dock

The Dock should restart automatically. Test the external display again.

Keep Displays have separate Spaces enabled during these tests. 

I would not change the display arrangement or delete Dock preferences yet. 

If the Dock still does not appear after these tests, the next step would be to isolate whether the problem occurs specifically with the combination of macOS 27, the M1 MacBook Air, the external display being above the built-in display, and a left-positioned auto-hidden Dock.

Apple documentation: Change Desktop & Dock settings on Mac and Connect displays to your Mac

To make the auto-hiding Dock appear on an extended external display, you need to designate that monitor as your Main Display or forcefully trigger the Dock to re-index the new screen configuration.

Fix the Dock on the External Display

• Open System Settings from the Apple menu.
• Go to Displays.
• Click Arrange.
• Drag the small white menu bar from your MacBook's built-in screen onto the external monitor box to make it the primary display, or click the target display and choose Main Display.

• Alternatively, press Option + Command + D twice to toggle auto-hide off and back on to force the Dock to refresh across active spaces

More info: Displays settings on Mac – Apple Support

Thank you for all the suggestions, unfortunately nothing helped.

Displays have separate Spaces is enabled. I've tried to reset it (and logout, login) - it didn't help.

Temporarily turn Automatically hide and show the Dock off - the dock doesn't appear on the external display, only on main display.

Killall Dock - no change

Option + Cmd + D - the dock is shown on main screen only.

When I moved the bar in Arrange Displays menu (set main to the top screen) - then the Dock appears only on the top screen, but not on the macbook.

My top screen (extended) is 4k monitor Asus PG27UCDM.

I've also tried to shift the bottom screen on the Arrange Display to move it to the right (so right sides are aligned) or past it - it also didn't help.

To conclude, despite Apple statement that when Displays have separate Spaces is enabled, the Dock should be available on all displays. (Apple Support) - it is not available on both in my case, just the main display.