I agree with you, it’s ridiculous.
you should be able to put in a voucher number and use the credit how you wish.
I too have a credit from Qantas and the booking was originally for 3 people, now 1 of those people is my elderly mum and she doesn’t want to fly now but I can’t use the voucher unless I book for all 3 of us.
Mira ridiculous. Like you, I paid for all flights with my credit card so I should be able to use the credit how I like.