OBJECTION! I have the evidence right here!
Exhibit A: The enemy archers clearly shoot at my dismounted troops but don't shoot at me(mounted)! We are at max distance for this map!
Exhibit B: Only the ballisteer and the 1 Xbow man fire at me!
Exhibit C: All quite and peaceful, until I dismount my horse then the sky darkens with militia arrows! As described by the OP, they continue to fire on me after I re-mount my horse.
(from un-modded 1.5.7 beta)
Something goofy is going on! I have reported already my experiences with ranged units often not fireing on HA and Cav when I expect them too and in other forms being goofy about it! I have seen other report this type of problem too! Something is certainly wrong with their AI.