I just recenty noticed this bug and experimented so i removed all AOE increase runes i use.
1) The Blast radius perk will increase AOE skills by 50% while increasing cast cost by 300%
2) Lava armor's default Aoe is 5m, Toggle cost : 5
3) Applying the 20% pre-requisite perk to get Blast radius increases the Aoe from 5m to 8.5, and cast cost of 5 to 20
Bug : When lava armor is on, Blast radius effect is off. When lava armor is off, Blast radius effect is off.
A. This bug only activates when you have Blast perk.
B. Bug affects Absorption shield too, but not Warrior's will