Lancers attack another cavalry unit with attached leader and defending player uses the Leader Orders a Fall Back tactician card-
Do the lancers get to reroll the flags or does the card effect of "only unit symbols will cause a hit" override it?
I am uncertain on the answer here, but a relevant comparison is Elephants in Ancients, since Elephants re-roll Swords.
The FAQ states " Only symbols that match the evading unit will score a hit when a unit is evading, flags, swords and leader symbols and other unit type symbols will all miss. In addition, If a unit evades an attack from an elephant, since sword rolls "miss," the elephant does not re-roll sword rolls."
"Note the number of initial flags rolled in the initial melee and add this number to the number of flags rolled on the re-roll. Total number of flags rolled, plus all unit hits and sabers rolled are then applied to the defending unit."
So it would seem to me that you roll the dice, rerolling flags as neccesary, add it all up then apply the results to the target minus flags and swords as per the Tactician Card.