I see the same issue (using Chrome on Win10).
After examining the $:/core/ui/Buttons/* shadow definitions, I note that some (but not all) have <$text text=" "/> preceding the button caption text. This includes edit, clone, permalink, permaview, open-window, close-others, fold-others, and fold. However, in contrast, the close, info, new-here, new-journal, and export-tiddler buttons do not have this explicit space preceding their caption text, but still seem to display properly.
Following this pattern, the fix would be to add <$text text=" "/> to the delete button definition.
-e