Why Isn't the Dock Showing/Hiding Correctly
I am using 10.14.6 also, and have noticed no issues with the dock on my computer, except I just use Auto-Hide for the dock so it disappears until I need it. Whenever I move the mouse to either screen the dock pops up -- but this might because there is still only one visible instance of the dock.

But I noticed something similar to what you describe on a coworker's 27" iMac the other day. 

On the second monitor was the Mail app and Finder was the active app on the primary screen. The dock was on the screen with the Mail app. Moving the Mail app to the primary screen the dock moved.

I don't know what the rules are for the dock, but if it gets stuck try toggling on/off hiding. (Right-click on the separator between standard an recent apps, then select "Turn Hiding On/Off").