This is a known behavior that is being considered as an enhancement request and in review for possible change in a future release of Applications Manager.
There are a couple of solutions:
Ensure that a past Calendar date exist when making a change to a Calendar.
Manually recalculate the next run date using a function that calculates all next run times for all schedules.
This can be done by logging into the Applications Manager database as the Application Manager DB user and running the following: