brundlefly
Raise the Threshold slider to 100%.

For tracks to stay in sync, they have to be using the same set of markers. Merge and Lock will apply all enabled markers to all selected tracks. This simplifies the process when you have markers across several tracks that need to be applied to all other tracks. In your case, with just one track with active markers, Apply is just as easy.

And the "Lock" part of Merge and Lock doesn't really have anything to do with markers. It just locks the clip positions to prevent inadvertently moving a clip relative to one with which it's sharing markers.