Mine is also gone. Have enquired.
It’s a known issue; given the nature of what’s currently going on in the world, it may take some time to be rectified, and so please be patient.
I filled out the web form on the 2000AD Shop and they then reinstated the discount - in case anyone else is still having issues. It took them a few days to get round to it though because of the 🦠.
My voucher disappeared so I sent a message through support.2000ad.com and they restored it within a few days. Of course, it's the 31st (and a bank holiday) now, but it can't hurt to ask.