I had a simpler issue with dispatch time where by some of my listing changed from 1 day dispatch to Sameday Dispatch. I did the same as you and used bulk edit which like you it worked for a few days and then I had a sale at 10pm and the dispatch time was back to same day, which was not possible to find a post office open at that time. Spoke to eBay next day and they had no answer but with 24 hours all my listing were back to 1 day dispatch and I've no issues since then.
If it keeps happening then maybe you need to contact eBay this way if a buyer complains you can tell them you have reported the issue and then as eBay to remove any defect on your account.