Not sure if this is the reason for your problem, but I think you have to write the data first, then enable the time change bit, you can use a one shot to avoid having to turn it of, as it can be too fast for the PLC, and use a timer in the PLC to turn that signal off after a while.
Private Sub SystemEvents_TimeChanged(ByVal sender As Object, ByVal e As EventArgs)
'Occurs on Time interval.
Dim time As TimeserverTime = NTPTime.GetTimeServerTime(DateTime.Now.Year)
If localZone.IsDaylightSavingTime(DateTime.Now) Then
' Time Interval ...... If daylight.Start = DateTime.Now Or daylight.End = DateTime.Now Then
' Time Interval Write DST occur write to PLC
PLC.Write("N16:0", DateTime.Now.Hour)
PLC.Write("N16:1", DateTime.Now.Minute)
PLC.Write("N16:2", DateTime.Now.Second)
PLC.Write("B3:0/0, 1) 'TimeChange Enable bit
'End If
End If
End Sub