Mac Dock jumps to DisplayLink or USB monitors

Short answer: DisplayLink and other USB display adapters add screens through software, and those screens often come back a few seconds after the Mac wakes. In that gap the Dock settles on another screen. Set the right main display, keep DisplayLink Manager current, and lock the Dock to one display.

Why DisplayLink screens behave differently

A DisplayLink dock or adapter drives monitors over USB with the DisplayLink Manager app, not through the Mac's own graphics outputs. That is how base M-series MacBooks run more external screens than Apple supports natively. It also means:

After sleep or a cable bump, the USB screens reappear later than the built-in one. The Dock lands on the screen that was there first.
When the adapter reconnects, macOS may rebuild the arrangement and change which screen is main.
DisplayLink Manager needs Screen Recording permission to draw. If that was turned off, the screens go blank or never appear.

Fixes, in order

Update DisplayLink Manager from Synaptics' download page, and check it has Screen Recording permission in System Settings › Privacy & Security.
Set the main display in System Settings › Displays › Arrange by dragging the menu bar to the screen you want. If it keeps resetting, choose a screen on a native port (HDMI or USB-C/Thunderbolt direct) as main.
Move the Dock to a side that only one screen has free. See ultrawide and three-monitor setups.
Plug the adapter into the same port each time. A different port can look like a new screen to macOS.

Lock the Dock to one display

None of the fixes above stops the Dock moving when the pointer pushes against another screen's edge. Still Dock's One display lock keeps the Dock on the screen you choose, DisplayLink screens included, in both the direct download and the Mac App Store version (Pro).

Dock changes size too?

USB screens often run at a different scaling from the laptop panel, so the same Dock looks a different size on each. See Dock resizes on an external monitor.

Common questions

Is this a DisplayLink bug?

Partly. The Dock following the pointer is standard macOS. DisplayLink's later wake makes the after-sleep jump more common.

Does clamshell mode help?

With the lid shut the built-in screen is gone, so there is one fewer place for the Dock. On wake the external screens may still return one at a time.

Will killall Dock bring it back?

It restarts the Dock on the main display. See what killall Dock does.