I have been playing Germany 1933 its currently October 1939, Poland is gone and yes my IC dropped when war broke out.
To give you my take on this Peacetime vs Wartime Industry. I notice that the ALL the Allies take Peacetime and the Axis Wartime. So how I read the information in the tech tree and how I relate it to the game/historical factor is this.
Peacetime gets the nice +15% research and you cant built "UP" till war hits. Makes sense to me as the economy is not suited to mass production during peacetime but well suited to research and develop technologies needed for war. So once war does come your industry should then gear up. Kind of reminds me of how the USA is handicapped with its IC in AoD prior to war, because if it was allowed full capacity it would just smoke everyone!
Wartime Industry gets a +18% Peacetime IC modifier meaning "BUILD BUILD BUILD" that war machine. As I see it your not at war and your industry is geared to build prior to war so the IC modifier gives you that advantage to build up prior to war so you can make quick work of those peaceful countries. Once at war it diminishes a little not a lot but keeps your war machine still viable till you grab more factories from all your victories.
Now thats how I take this tech tree information to stand out. I did not built lots of factories as Germany prior to 1939 maybe about 8 or 10 all told in various cities mostly Berlin though. I built "infrastructure because in AoD thats gold to do and so far its helped with my resources alot, so I built more of that in many provinces which yielded some high resources to keep my industry afloat without help of trading.
I understand the confusion and the "bug" if you want to call it this, but its not too hard to get around it, granted I have not played allies yet, so I could be wrong and if I am so sorry, its just how I precieve the "tech tree" to work for both Peacetime and Wartime Industry.
Well I hope I didnt confuse anyone because this is how I take the tech tree to go and how it would apply to game though many have good points on this issue.