I have a problem with tax calculations where a Discount Coupon and a Group Discount apply...

Sub-Total: $10.00
Shipping Not Applicable: $0.00
Discount Coupon (25%): -$2.50
Group Discount (10%): -$0.72
GST (10%): $0.75
Total: $7.53

The GST is calculating with the Discount Coupon but not the Group Discount.

The Group Discount options are:

Include Shipping
true

Include Tax
false

Re-calculate Tax
Credit Note

Tax Class
GST

And the Discount Coupon options are;

Include Shipping
true

Include Tax
false

Re-calculate Tax
Credit Note

Tax Class
GST

Zen Cart version is 1.3.0.1.

On it's own the Group Discount works;

Sub-Total: $10.00
Shipping Not Applicable: $0.00
Group Discount (10%): -$1.00
GST (10%): $1.00
Total: $10.00

As does the Discount Coupon on it's own;

Sub-Total: $10.00
Shipping Not Applicable: $0.00
Discount Coupon (25%): -$2.50
GST (10%): $0.75
Total: $8.25

Any Clues?