So these sorts of errors occur regularly on the JQ website and have so for many years. The scripting of the fare is not formatted correctly where often meals come free or a $10 or $15 voucher ( depending on route) can be added on with no additional payment.
An example I came across late last year, was both myself & my +1 were booked on an “economy max” bundle from MEL> BKK where we could Choose two meals on this segment segment as per the flight that stimulates that Two meals on this segment are provided. In additional, one week prior to departure when attempting to change our meal selection , 8 realised I could add on a $15 voucher each at no additional cost.
So we had the two stipulated meals, plus a $15 voucher to utilise, which equaled plenty time of drinks for both us.
Yet, on the return journey I could not add on the $15 voucher without the website attempting to charge me for the additional voucher.
So this error occurs on some segment, but not on all segments.