This appears to possibly be broken again, at least for me. It seems to register everything correctly. I can switch the buffer chest option for example, and the car reports back if it is or isn't using buffer chests. Also, the "trash" slots work. It just won't refill the logistics (locked) slots from any chest, let alone a buffer chest as would seem to be indicated here. As of writing this I'm on 0.18.40 but knowing there is an update out there, will probably shut down instead of just pausing when I stop playing for the night. If it suddenly starts working on 0.18.42 (or whatever it is tomorrow the rate they've been updating the past few days) I'll let you know.
On a side note: if it was working, should it work with "regular" chests or will it only work with buffer chests? My logistics network is built around storage chests. The only reason I have any buffer chests is for testing like this and they normally live in my character inventory and aren't actively used. I'd like to keep it that way at least for now.