did you add it on the existing booking, or do you have a separate booking and apply the voucher across?

also for long haul, taxes/fees/charges for a return is not equal to TFC if booked as one ways just added up. in order to know what the true TFC would be you would have to price up a cash booking for LHR-SIN/KUL-LHR and look what it shows in the breakdown.

and lastly i suspect there is some RFS complication here as well which may be causing an issue.