Prior to 1.6.25.0 I did not have this issue - many/most of my flights would use the live historic time slider adjusted to a different time of day within the past 24 hours, but beginning with 1.6.25.0 this is no longer respected. Flights always spawn in approximately 5 minutes behind the current live time with real time disabled.
Selected:
Spawned:
Also worth noting - the time on the live/historic time slider can be perfectly on 12:00 but it will reflect 11:52 (+/- a minute or two, as shown above).