the change was comming with the new movement system in 1.9.0 i suspect the 360 degree inputI dont remember which patch was that changed it
" Combat
- Overhauled the AI movement system through a range of physics adjustments, better situational understanding, and manners (waiting on the allies ahead) and by implementing a partial, 360-degree input for the AI. This notably improves issues with troop congestion (jittering), helps the AI deal with difficult terrain and avoid siege engines. "
i have seen an ai circle formation all turned their back on my front. as no shields got involved, my archers could easily mow them down. note that i wasnt having any sort of distraction behind their line or intended anything else to cause ai troops turning their back on me.