Same for me in 5.6.0p1

The same...

+1

5.5.2p1 - Still not works
5.4.5 - Still works

I need exactly ZlibStream, which is not in System.IO.Compression

this happens just when decompressing, as workaround you should be able to use this:

string savefile = "";
using (var decompressor = new System.IO.Compression.GZipStream(new MemoryStream(bytes), CompressionMode.Decompress)) {
using (var sr = new StreamReader(decompressor)) {
savefile = sr.ReadToEnd();
}
}

or something similar

Same for me in 5.5.0f3