A schedule is created to run daily and a time interval is set that covers the entire day, 12:00 AM to 11:59 PM and the schedule executes every minute. After saving the schedule and editing it again, the schedule reverts to use a timer set to run every minute.
This is normal behavior. The system detects that using a time interval that covers the entire day, 12:00 AM to 11:59 PM and that executes every minute is the same as a timer set to run every minute and so it reverts to use a timer.
Resolving the problem
To avoid this behavior, set two time intervals instead of one, as per example:
12:00 AM - 11:55 PM Every 1 Min(s)
11:55 PM - 11:59 PM Every 1 Min(s)