I work in a division where old timers get paid time and a half on Sundays. So everybody's paystub has a listing for Sunday pay whether it's time and a half or not. I've been full time for almost 30 years. I don't get time and a half on Sundays. I missed it by two years. Anyway, my paystub usually says 8 hours Sunday pay and 32 hours regular pay. Lately it's been saying 8.5 hours Sunday pay and 31.5 hours regular pay. It's still 40 hours, but it just seems fishy for some reason.
I'm grandfathered into time and a third as well. You should be honored this. If you are getting less than this or have had it compeletly removed contact your union or store level to fix it. The only time it would be removed is if you transfer out of the union and you loose your seniority.
You need to sort this out at the store level. They can work this out. If you are not given time and a third you need to file a pay grievance. Unless you quit and got stripped of it you should get it.