macOS 27 Dock stuck on MacBook Pro display with external primary monitor

The issue you are experiencing is by design due to a structural limitation in macOS regarding vertically stacked displays.

When monitors are stacked vertically, macOS defines the "bottom edge of the screen" as the bottom of the lowest physical display (your MacBook Pro). Because the bottom edge of your external monitor is seamlessly connected to the top edge of your MacBook screen in the arrangement settings, dragging your cursor to the bottom of the external monitor simply moves the cursor onto the MacBook display rather than triggering the "summon Dock" boundary. 

Because this is tied to how the operating system handles virtual coordinate spaces, traditional system wipes, reinstalls, and account recreations will not fix it.

How to Get the Dock Back on Your Top Monitor

To keep your physical arrangement exactly as it is, apply one of the following software workarounds:

Fix 1: Offset the Monitors

• Open System Settings > Displays and click Arrange.
• Move one of the displays to the side (corner to corner)
• Click Done.
• Move your mouse to that specific protruding bottom corner of your BenQ monitor and push downward. The Dock will immediately jump to the top screen. 

Here's an example from my Mac that's still on Tahoe, the internal display is on the bottom in the Arrange tab.

Fix 2: Change the Dock Position to the Side

If you move the Dock away from the bottom entirely, it breaks the vertical boundary constraint.

• Go to System Settings > Desktop & Dock.
• Change Position on screen from Bottom to Left or Right.
• Since your displays are stacked vertically, the left and right sides of your BenQ monitor now have isolated, distinct boundaries. You can easily invoke the Dock on the top screen by moving your cursor to the designated side. 

Fix 3: Lock the Dock and Sacrifice Separate Spaces

If you want the Dock stuck to the top monitor permanently without it ever jumping down, you can disable the multi-monitor independent workspace feature. 

• Go to System Settings > Desktop & Dock.
• Scroll down to the Mission Control section and toggle off "Displays have separate Spaces".
• Log out of your macOS account and log back in to apply changes.
• Note: This forces the Dock to stay on your primary display (the BenQ). However, it means when you make an app full-screen on one monitor, the other monitor will blank out or share the space

I can't fault Neuroanatomist's excellent explanation. I am only adding a divergent explanation that I use to explain where it ends up, in case you find that more accessible.

I use this rule of thumb for Extended Desktop and the Dock:

The Dock will appear on the designated edge of the primary display, UNLESS --

you have extended that edge of the extended desktop by placing another display (anywhere) along that edge. In that case, the Dock will appear on the displayable portion of the Extended Desktop.

so let's try that out with your setup:

the dock will appear on the bottom edge of the primary display Unless

you have extended the bottom of the Extended desktop by placing another display along the bottom, (which you have) in which case the Dock will appear on the displayable portion of the extended desktop (the bottom of the bottom display in this case).

Thank you so much for this detailed response 🙏

What bothers me is that it used to work before, so a change under the hood in macOS has resulted in what I consider a regression.

I’m torn between moving the dock to the top and using DockLock Plus, I don’t mind buying it, but spending 45 euros to fix an Apple “bug”...

Thanks again 🙏

<< What bothers me is that it used to work before. >>

I disagree.

This is the way it has worked for a very long time, unless you changed your setup somehow.

I didn't change a thing…