Since the mill received a second work slot, they don’t offload their flour anymore. Instead, when the storage of the mill is filled with flour, they go “unable to work” even though there is an empty granary nearby that accepts flour.
This was from a long running save so I have not tested if this is still an issue with games that are started with the newest patch.
I will check it again when I am able, maybe it will be solved upon reloading.
Edit: Just reloaded the save and the issue persists, but it is not constant. When the mill is full with flour but not with grain (and the barrel icon doesn’t appear), they drop off flour just fine. But when there is a lot of flour and a lot of grain, sometimes they stop working, and the barrel icon is over the mill.
The issue seems to be that if the threshold is reached where the barrel icon shows, they don’t drop off flour anymore but they do drop it off before that. I had the filled granary right next to the mill, maybe the very short walk to supply the mill is causing problems.