This is in MrProject as well as a Planner bug so its a legacy bug.

Basically do the following,

Open an existing project,
Go to gantt view,
Click a task and right-mouse Edit task,
Go to your resource assignments and alter the units allocated
from 100 to e.g. 75.
The resource units in the gantt row doesn't actually update though it
actually does the job i.e. the line will change size.
Once you do an assignment then they all update.

I do have a simple fix which I'll forward a patch for. 
Look at the gantt_row_ensure_layout () in planner-gantt-row.c
and make sure that gantt_row_update_resources (row) is never
missed out (moved it down a line). This seems to fix the problem.

@@ -602,9 +605,8 @@ gantt_row_ensure_layout (PlannerGanttRow
 	if (row->priv->layout == NULL) {
 		row->priv->layout = gtk_widget_create_pango_layout (
 			GTK_WIDGET (GNOME_CANVAS_ITEM (row)->canvas), NULL);
-
-		gantt_row_update_resources (row);
 	}
+	gantt_row_update_resources (row);
 }

(the line offset may vary as I'm doing other patchs to that right now).