So, it is not a bug. Do not investigate it.I've noticed that any damage-display-anomaly means this hit will be with a crit. This feature is abusable for CV cause make you able to distribute damage perfectly) For example after each move you can check predicted mage's damage to detect her critical-ready status to eleminate a fat enemy reg this turn. It helps to CV through some 4skull battles in FBS.
So, no need to complain about this in-player's-favor feature