How to lock dock to a selected monitor?

When I use the Displays settings to assign the dock to my middle monitor, it keeps moving around to other monitors.

I've already read through the suggestions on threads here. Things I've tried include:

• Using the drag-down motion to pop the dock back onto the middle monitor (it seems to work about 2 out of 10 times, but only when I don't want it to)
• Setting "Displays have separate Spaces" and logging out
• Moving the dock back to the middle monitor via settings (when I switch it to this monitor, it automatically moves back to the MacBook native display)

This is extremely frustrating and it seems like every "fix" is temporary or broken, and the threads are all locked. How do I permanently lock the dock to one monitor so it never moves again?