Scheduled events are interval-based, the trigger you set at 12:30 has a date attached to it which is probably in another timezone (i.e. within the DST period), the event will simply run every 24 hours.
Edit after comment: No a restart wouldn't do a thing since the scheduled event is still scheduled within the DST period. You could change the start date of the scheduled event but then you'd have to do that every half a year.
If you must schedule the event at a certain time instead of an interval a better solution would be to make 2 events and have 1 for the DST period and 1 for the non-DST period of the year. Then check the date in the flow to decide if it should run.
Also see https://forum.mendix.com/questions/3700/Scheduled-events-start-one-hour-late-since-start-of-Daylight-Saving-Time
I already have a ticket about this subject from june 2011 :) (ticket 8688 Boolean checkbox for DST correction on scheduled events) See also this post Server time changes summer / winter
I solved it by using two microflows, but I was under the impression that 4.2.1 could have a different sollution. In the settings we now have the option to set the scheduled event time zone. I was under the impression that if I would set the scheduled event time zone on Amsterdam for instance and use server time in the scheduled event the corresponding microflow would run on the same time (lets say 2300 hours) amsterdam time no matter if it was summer or winter. May be Bas could clarify. Otherwise my sollution for a dst checkbox on the scheduled event would still stand.
We had a similar problem supporting users from different time zones. We solved the issue by schudeling the event hourly. Within the microflow check if the local time of the user already reaches 12.30 and only then run your planned actions.
Ronald Catersels pointed us in the right direction of using the Java action 'executeMicroflowAsUser' from the 'Community Commons Function Library'.
This way you don't have to worry about DST.