I have the same issue come to cancel my membership on 18th december 2023, was offered the $30 gift card.  I Renewed and after two weeks didnt get a voucher.

I sent them all the screenshots as evidence.

Ive escalated multiple times they keep telling me to wait a few days. First they apologised for the delay offered me a $10 voucher for the delay and told me it was escalated and would be a couple days. That was 3 weeks ago.

Been three weeks ive called and chatted to them at least 6 times keep getting told that its been escalated, and still keep getting told to wait a  few days.

Dont trust them and dont renew your eBay plus account if your offered a gift card.

I accepted the $30 voucher for renewal offer (I was initially going to let it lapse), and took a screenshot of the offer just in case. Then, when my renewal date came around, the membership didn’t actually auto renew like eBay had said it would in their renewal reminder emails. I didn’t get any notification of failed payment or anything. Now, when contacting eBay support, they just say I have to pay the full $49 to sign up again and they will not send me the $30 voucher. 
It feels dodgy and they are claiming I cancelled my membership prior to the renewal date, but I definitely didn’t, and I have proof that I accepted the renewal with $30 voucher offer.