The answer was found long long ago.
For example was worked into a Flotilla.cfg by WB's Mod "XXI_1939.rar" (when i remember correctly by WB Wredford Braun).
'Simply' put in the missing Lines for each used Month.
The chance to stumble across this issue is very thin.
But now it is officially known where it comes from, as it is written down.
Greetings.