First, sometimes the “attack drums” buff is active but the icon doesn’t show above the unit. See the start of the clip and the selected mehter.
Second, sometimes multiple buffs get activated and they’re cycled through very quickly, since only one is supposed to be active at a time. I haven’t done testing to see what actually happens, if it depends on the particular fraction of the second when the unit attacks or is attacked whether a particular buff is applied. In the clip, all three mehters start with “attack drums” active. I set the second mehter to “melee defense drums”, then the third to “ranged defense drums”, and finally the first to “ranged defense drums”, which causes them to rapidly switch between the two buffs. Shortly thereafter another mehter pops out of the stables with “attack drums” active and they cycle through all three buffs. (This applies to other units that are affected by the buffs, not just the mehters themselves – that’s how I first noticed it, clicking on an enemy’s unit to see what upgrades they had and noticing this rapid cycling.)