We highly recommend you perform a regular check on Apps Update and take actions to update periodically.
Check for App Update
In Admin Console, go to App > App Management
Check the far right column, and note the Outdated indicator
Tap Outdated icon
and a screen pops up
Also, if Missing indicator is non-zero, tap Missing icon to check the devices list
Update App
Tap "⋯ Action" button on the top left corner
Select VPP Install All if the app was initially VPP installed, otherwise select Install All