It's been bugged since update 1.9 and it still isn't fixed. As of now, you can't unload partial bales (2500l, 5000l and 7500l) from chamber 2.

It's bugged on Xbox one as well. Can't unload partial bale from chamber 2. It worked pre-patch 1.9.

The John Deere is bugged as well. Unfolding a new machine is backwards, can't unload partial bales (2500l, 5000l or 7500l) from chamber 2 and the engine keeps on running if you jump out.