Actually this is a problem of the underlying font rendering engine of Java. With the given font size a dot just will not be displayed. Choosing a larger font size for the dots wouldn't work due to scaling issues with the surroundings. So the only possible solution would be to deactivate zoom factor 1 - which I'd prefer not to do.

Just a thought, but what if you changed the symbol when at a very low zoom factor? And the same could be applied to any other symbols that fail to display at very low zoom factors.