"isn't it related to the turn range of the surrender event? If so it's not a bug but rather a WAD. it makes sense to me, it's not like everyone is going to stop fighting once the capital falls. a 2,3 turns delay isn't that unrealistic."
I fear not: there isn't a delay built in - Paris being occupied by the Axis at the start of a turn (Event 82) should trigger the withdrawal of the French (Event 85).
The reason there isn't a delay - which, as you say, wouldn't be unreasonable - is because the Fall of France triggers other Events, like the formation of Vichy and the Axis TO to mobilise Spain (and that in turn triggers others) so a delay - perhaps allowing a strong Allied player to retake Paris in the interval - would screw up the Event sequencing.
"Now you can start working on improving naval combat" Amen to that. What I'd really like to see is ships being treated more like air bomber units, but obviously limited to affecting the sea hexes only, i.e. they can still physically be moved but then be set to 'combat support' or 'interdiction' - I imagine that 'naval superiority' might be too difficult? - and either provide support to naval landings/defense as appropriate or, if on interdiction, have a %age chance of striking other naval or embarked land units moving within their range (which would need to be vastly increased). Land units forced to disembark at sea would have to sink.
I'd imagine that this is a bit ambitious, but it would be better than the giant floating artillery batteries that we have at present, at least for the larger scale scenarios like EA.