So if you look at Slow's status entry, you'll see this:
Code
new entry "SLOWED"
type "StatusData"
data "StatusType" "CONSUME"
data "ForGameMaster" "Yes"
data "InitiateCombat" "No"
data "DisplayName" "SLOWED_DisplayName"
data "DisplayNameRef" "Slowed"
data "Description" "SLOWED_Description"
data "DescriptionRef" "|Reduced Movement speed by [1]%. Cured by Hasted, Adrenaline.|"
data "DescriptionParams" "Stats:Stats_Slowed:MovementSpeedBoost"
data "Icon" "statIcons_Slowed"
data "FormatColor" "Red"
data "StatusEffect" "RS3_FX_GP_Status_Slowed_01"
data "SavingThrow" "MagicArmor"
data "ImmuneFlag" "SlowedImmunity"
data "StatsId" "Stats_Slowed"
data "StackId" "Stack_Slowed"

Specifically: data "SavingThrow" "MagicArmor"
A way to get around this would be to create your own Slow (which is preferrable than overriding the original), and remove the saving throw.

LaughingLeader's DOS2 Mods