If you examine the file behind the Symbols pop-up, you will find it is making 18 additional file calls for CSS and Javascript (http://downloadsv12.xara.com/web/cha...character.html).
I believe the Xara Desktop application is failing to refresh these properly and if the download sequence falters, you end up with missing displays.
it is also bad in the main Desktop application as some of the furniture fails to be rendered and you get white artefacts glaring out at you.

The page design has each symbol with its Title that will appear on a symbol roll-over and is used in the Search box.
Xara has also included the data-unicode parameter but fails to allow a search on that.
Xara could have included a right-click "Find similar" or proper Wildcarding to allow intelligent searching, but there you go, that's Xara.

Until those will the problem raise it back to Xara, all you can do it try pressing Ctrl+F5, F5 or Ctrl+Shift+T when the Symbol window has focus to force a refresh or redraw.

You can also use http://downloadsv12.xara.com/web/cha...character.html to identify the actual symbol you are after and then type its Title into the Symbol search box.
Perhaps the searched symbols will then display.

Acorn