I've encountered this bug a couple times before over the past couple years, and once again just today. I wasn't sure where to report it though, since the Github issue tracker for itch seems to be full of spam.

The steps to reproduce (it's inconsistent):

1. Edit an existing but expired sale (Maybe it could be reproduced with new sales, not sure)

2. Add items to sale if needed (I've only tested with multiple items)

3. Set start/end dates in the future (start date a minute or two from now is OK)

4. Wait for the sale to start, then view sale page to verify it started

5. Visit the page of one of the items included in the sale to check if it is actually discounted

If the issue is successfully reproduced, the sale page will look active and the items will appear to be on sale from there, with a percentage off indicated in each project's thumbnail. However, clicking on any of the thumbnails to visit a page of that individual item will show that the item is still at full price, with no "View bundle" link, no "On Sale! XX% Off!" text, and presumably no email sent to followers.

It doesn't happen every time. Previously I chalked it up to possible itch.io downtime, but this time it occurred while itch was definitely not down.

Edit: I forgot to note that it's not simply a matter of waiting for it to fix itself, or for things to process behind the scenes. It never does, and when the issue occurs sometimes I only notice several days later that nothing actually went on sale. If the sale works, then the prices are indeed discounted instantly.