This happened to me back in October last year. When the offsite ads programme began, I de-activated the left-in-cart coupon with 10% off as it was set to run until end December 2021 and I did not want to be hit with an offsite ad fee and a 10% coupon code. Somebody used that code in October 2022. Even if I had not de-activated it previously, it was ten months past the date when it was set to expire anyway. Like you, I honoured the discount, but had to go into settings and de-activate a code which was still active almost a year after its original expiry date, but which had been cancelled sooner by me. I should have contacted support, but I just could not be bothered, since contacting them is so complicated, and even though it was over 20€ I'd 'lost'.
