We tested several attacks of fused sylphs against wind ranger, It seems:
When wind ranger buffs opponent or shields:
1) MATK reduction is applied - so average ATK hit is lower
2) MDMG reduction is not applied - so no change in damage
So while ranger is perfect against MATK attacks, fused sylphs attacks are reduced only partially - ie MATK part of attack is reduced only on opponent side (the attacker), not on damage side (the defender).
When wind ranger buffs opponent or shields:
1) MATK reduction is applied - so average ATK hit is lower
2) MDMG reduction is not applied - so no change in damage
So while ranger is perfect against MATK attacks, fused sylphs attacks are reduced only partially - ie MATK part of attack is reduced only on opponent side (the attacker), not on damage side (the defender).