Lock time zone setting in the S Planner says:
Lock event times and dates to your selected time zone below. Times and
dates will not change even if you move to another time zone.
This means that the event retains the same timezone it was originally created with. However, the display will be adjusted according to the locked timezone.
e.g. If you were originally in GMT+3, and you created an event using GMT+3 at 10:00, and then locked the timezone at GMT+4, then you will see that the time has changed to GMT+4 11:00 to reflect the selected timezone. Internally, this is still GMT+3 10:00.
New events that you create will be based on the locked timezone.