Nothing wrong with it at all. Some companies decide to show GST inclusive amounts on their bills, others show GST exclusive. In this case Vodafone has decided to show the GST exclusive amount so makes sense they would exclude GST from the discount amount (as they would for all other amounts showing on their bills). Move on, nothing to see here.
GST inclusive statements can show rounding anomalies too. Presentation can get a bit tricky when it comes to line item tax vs summary tax.
Ideally billing apps should be able to bill tax according to the target customer type.