This seems related with the difference between zlib format and raw format. When you do raw inflate, you have to inflateSetDictionary before any inflate. You cannot rely on the first inflate to return Z_NEED_DICT and then do inflateSetDictionary.

Although I have the solution with experiment, but there is nothing can be found in the official zlib documentation. It is rather vague. I'll make more research and submit a patch later..

Right now, you can use zlib format (positive wbits) if it can achieve your goal. It's behaviour is right.