I thought it might be some weird interaction with part of the supply chain to mainland France being part of the Hessen network instead of French, so I built a depot in Hessen land under French control, and that depot filled up with supplies just fine, but they still aren't being passed on to Prussia. I have no clue how I could ever possibly be expected to make it all the way to Poland under these conditions. Am I doing something wrong?
Here's my overall situation. As you can see, I have a full line of supply from France to Berlin but no supplies stockpiled anywhere in Prussia (Stettin was just conquered, so those are captured supplies):

Here's my newly made depot in Hessen. It's working perfectly normally and seems to be sending and receiving thousands of supplies per turn:

And here's Magdeburg, which like everywhere else (including Hamburg) in occupied Prussia is not sending or receiving any supplies whatsoever:

Is this a bug? Do occupied depots not work and I should be building French depots in occupied territory? I have no idea what's going on here.
Playing on patch 1.03.