It is an issue with how they are displayed by the game since some update.
You can enter the editor, check that it is set to use SD assets (to avoid data corruption for mixed and SD campaigns), open Modules - Campaign Editor, open the campaign in question, remove the corresponding symbols, and save it.
If you are using the latest version of the game through Battle.net, don't forget to post about it on Blizzard forums so it has a higher chance to get fixed.