Google Maps - Dynamic Markers disappear on page refresh

I have some simple dynamic markers displayed on a page. If I refresh the browser then the markers disappear.

By opening a brand new browser window and reopening the page then all is OK until I refresh the page again when they again disappear.

This has to be a browser session started after closing the original window - if I just open on incognito window using the original windows then the markers are not displayed.

Any thoughts ?