Mixing Steam API versions, issue in packaged

Hey there,
I currently try to get a Plugin to work that uses Steam API. Unreal is as of UE4.19 on Steam API v1.39. But Steam only has documentation for the newest API v1.42. There are also some changes in the API that we require. Exchanging the SteamAPI comming with the engine is no option. I was told by the Steam Dev Group that it basically does not matter, as the SteamAPI works with steam directly and not so much with the SDK binaries. So I included the header files (only the header files) for the new v1.42 API into the plugin.

Thing is, it works. I can use functions making use of the old API (1.39) and functions of the new API (1.42) and result are the same (at least for the functions I used for testing). But only as long as the game is run in Standalone (not packaged)

Packaging the game comes with multiple results.

First result: Only the functions working with the unreal included API (1.39) work correctly. Functions working on 1.42 do not. (Like GetNumberOfWorkshopItems returns 0 when it should return 2)

Second result (yes this was a luck shot): Including this simple function which relies on the 1.39 API in a FunctionLibrary of the Game-Module

	FString GetSteamMyName()
	{
		if (SteamAPI_Init())
		{
			const char* PersonaName = SteamFriends()->GetPersonaName();
                    return FString(PersonaName);
		}
		return FString();
	}

makes the 1.42 API functions work. But now the functions the 1.39 API return strage values.
(Like GetNumberOfWorkshopItems returns 2 with 1.42 and return 33 with 1.39).

It works in a not packaged game properly with both API versions. Whats the difference?
I gues it is something to do with linking as when not packaging, .dll files are build that can’t exclude anything as everything might be needed by the user of the dll. When packaging an executeable is build and thus stuff might have been omitted. Which would also explain why it suddenly starts to work when adding the GetSteamMyName function to the function library of the Game-Module, but no clue why it now breaks the 1.39 functions.

Anybody?