When you use characters as marker fills for a polygonal area there is no way of ensuring that the actual characters are visible in legend patch. Indeed it seems to entirely depend on where the legend is placed as to whether that characters will appear in the patch or not. The image below shows auto-generated patches for exactly the same symbol, which is useless without the "Ba" part visible. There should be a tick box to centre the patch on a user selected marker or the ability to "activate" the patch and manually move the symbol; to best display it in the legend.
