Discovered through repeated expereince in use.
This behaviour is a regression over 2.2
1) Write post #1.
2) Set publish date to be in the future Day 7
3) click publish
4) write post #2
5) set publish date to the same future date & time Day 7
6) click publish
7) open post #1 and re-set publish date to be day 14
8) click SAVE (there is no publish date)
on day 7 both items publish automatically
on day 7 post #2 published
on day 14 post #1 publishes
I haven't found one other than being sure you get the publish date right first time around