<GameModifier>
<ModType>Unit</ModType>
<Attribute>AdjustUnitStat</Attribute>
<StrVal>UnitStat_ResistBlunt</StrVal>
<StrVal2>UnitStat_IsOrc</StrVal2>
<Value>-1000</Value>
<Duration>-1</Duration>
<Effect>Y_Defend_Particle</Effect>
<vsLower>1</vsLower>
</GameModifier>
This modifier was in a spell, and applied to an enemy unit. The point was that if the unit later on would be attacked by an enemy unit that had a lower UnitStat_IsOrc, that unit would be able to deal +2000% blunt damage to the unit the spell was cast on.
The strangest thing happened, however... Even though the spell was applied correctly, and the effect played for a while, as soon as I hovered my mouse over the unit the debuff disappeared. I could hover over the unit in the unit list on the left and see the debuff, but if I hovered over it normally, the effect would immediately stop playing and the debuff would not be there. The offender is the <vsLower>1</vsLower>. If I removed that tag, the spell would work (although without the tag functionality). If I added it back in, the same problems appeared again.
First observed in: FE beta 0.77