I tried several things :
- First I believed it was because I dismounted some unit of cavalry, which I believed could have messed up the calculation of FF since dismounted cavalry can place them too. I tried mounting them back, no change. The problem didn't arise against AI either.
- Second guess, it could be because of the artillery, I put the bombarb on the river while still selecting the force to see how far I could place it. Artillery use the deployement zone of light units but can install FF despite having none by themselves in the Polish, Lithuanian and Russian army. I use it often to have all chariot fortifications two square further than normally (could be an exploit in itself). Maybe being on the river mess up the FF for the rest of the army ? I tried moving it but that didn't change anything. I haven't got a similar map in my other test.
- I tried to save and reload my turn to no avail.
In the end, I couldn't reproduce this problem against the AI so I'm not sure what causing this.