I've not explained it well probably but on the last upload 02 Feb 2022 - 13 cards accepted points - 1 failed (not noticed by me) That card would have expired in Jan 2022 so it makes sense that it failed
The only legal argument I could think off would be that they do not appear to have the expiry date checker on their platform that they say they do:
Text from their email of 20-04-2019 - I am writing to you regards the response below, as much as I appreciate your disappointment the Gift cards supplied have an expiry longer than the market average and in terms of use and taking in to consideration of the COVID 19 pandemic both brands within the Sainsburys group (Argos & Sainsburys) have been open for business throughout to be able to redeem and shop as normally as possible.
Argos Gift cards have a dynamic balance checker that include the expiry and how long you have left to redeem them, plus can be validated on your Argos.co.uk profile/account, Sainsburys Gift cards can be checked via My account on the Sainsburys page also.
I appreciate expiry of funds not used is disappointing, but we do clearly advertise the Terms and conditions both on our physical and eGift cards as well as our dedicated Gift card web pages. Sadly, we shall not be able to extend your Gift cards as we do not have the functionality to facilitate this.