What happens to an in-progress `7-day phased release` on the App Store when a newer version is published?
It would not immediately release 1.1.
If your publish 1.2 with immediate availability, then all users will be upgraded to 1.2 the next time the auto-update process on their decice kicks in. This means that both 1.0 and 1.1 users would be updated at the same time.
If you publish 1.2 with phased roll-out, then that phased roll out will run independently from the other one. I.e. 1.1 is not released immediate to the remaining users, nor are users left on 1.0 until they get 1.2.