If the code show is all that you do with the TextStyle variable then you did not initialize it properly. There is also the field RightToLeft which you did not fill, and thus might have the value 'true' which exactly causes this effect.
The best way to initialize TextStyle is first to assign to it the currently used values of the Canvas.TextStyle, and then to modifiy the fields that you want to:
var
TextStyle: TTextStyle;
...
TextStyle := Printer.Canvas.TextStyle;
TextStyle.SingleLine := false;
TextStyle.WordBreak := true;
Printer.Canvas.TextRect(TRect.Create(0, 0, Printer.PaperSize.Width, Printer.PaperSize.Height), 0, 0, '5.6/X', TextStyle);