Follow these steps:
Go to setting>battery>power master.
Inside power master window go to power master settings.
Toggle off the marked setting.
Go back>go to auto-start manager, under preloaded apps tab, toggle off 'all apps' switch and then toggle it back on (all preloaded apps will be autostarted after later steps).
Repeat the above steps under downloaded apps tab (all downloaded apps will be autostarted after next step) .
Restart the device. After restart, don't forget to check the ram usage and throw some cpu, gpu intensive load or play some graphics heavy games like pubg etc at max settings and check temperatures, open some apps and multi task. The performance, battery life, temperatures will be far better than before, see for youself....??
It is a zen UI related bug, overiding and denying android OS permission to autostart and manage apps in the background which adversely affect the device performance, battery life, temperatures etc....
Note: auto-started apps in the background will be cleared from the ram by android itself after use.
IF SOMEONE IS ABLE TO GET THIS INFORMATION TO ASUS TEAM'S NOTICE, IT WILL BE MUCH APPRECIATED, So they could fix all the bugs in the later software updates hopefully.....?.