Hi there, I have some updates. First, I tested in battle all the FMs on my campaign I can confirm that this routing issue does ONLY happen to Akrotatos (my FH). Secondly, I can also state that this very strange problem has to do with morale. My proof is based on the fact that when I commanded the army with a different general and I ordered some units to retreat the message on the unit state box said "marching, steady" and the units retreated from the field of battle normally. On the other hand, when using Akrotatos on the same army the message on the units said "routing, broken", and all units automatically routed (as expected).
Normal army:
Akrotatos' army
For additional information I also attached screenshots of Akrotatos' list of traits and ancillaries: