(Windows 10, Gramps 5.1.2.3)
In “Place coordinate gramplet view” if you change map provider through right-click menu the marker disappears.
For sure if you then go elsewhere in Gramps and come back to the view the marker reappears.
But as with other views in Geography category the markers don’t disappear when you change map provider perhaps should it be nice to fix that in “Place coordinate gramplet”.
Best regards.