Reason for battery drain in flight mode is the buggy phone modem in Samsung S8 Android 8 Oreo.
In case you experienced massive battery drain in flight mode, you can easily test yourself:
> Set preferred network mode to 2G/3G only, wait 2 minutes and then enable flight mode during night: You will experience only few battery drain, as you know from Android 7 Nougat.
Workaround: Flashing phone modem of Android 7 Nougat. But you will lose OTA Updates capabilities.
So maybe just test and give detailed Feedback to Samsung. If many users do, they have to notice this bug and will hopefully getting it fixed.