Announcement from ASUS on their forum, the ZenFone 4, ZenFone 5, ZenFone 6 and PadFone S will have another round of Android 5.0 Lollipop delay which was suppose to arrive in April then moved to mid of May.
According to ASUS, they found some issues during verification and decided to delay the update in order to provide better user experience. They are working on the related issues and will release the update ASAP but no ETA.
As for the ASUS PadFone Inifinity, due to limitation of the CPU (A86) the update has postponed the upgrade to Android 5.0 Lollipop and will provide better software upgrade based on Android 4.4 KitKat OS.
Source: ASUS Forum