In short, for some reason damage expected (showed by the game) is in all cases lower than actual damage done. In 10-seconds case when 1 second remains at the end of the spell no damage is done (though 1/3 should be I think). When 2 seconds are left some damage IS done, though not an amount I'd expect (2/3rds when damage was divisible by 3, 1/2 when it wasn't?),
I have also tried testing casting Concelhaut's Corrosive Siphon on a party member that either wears or does not wear Corrosion DR armor - and in both cases damage done was the same which suggests that DoT effects are not subject to Damage Reduction.
Bugs - Damage over Time issues
in Pillars of Eternity: Technical Support (Spoiler Warning!)
Posted
@Insolentius that is not exactly the case.
I have gathered my research on DoT spells (Concelhaut's Corrosive Siphon to be precise) in an StackExchange:Arcade answer: http://gaming.stackexchange.com/questions/211768/how-often-are-damage-over-time-effects-applied/211791#211791
In short, for some reason damage expected (showed by the game) is in all cases lower than actual damage done. In 10-seconds case when 1 second remains at the end of the spell no damage is done (though 1/3 should be I think). When 2 seconds are left some damage IS done, though not an amount I'd expect (2/3rds when damage was divisible by 3, 1/2 when it wasn't?),
I have also tried testing casting Concelhaut's Corrosive Siphon on a party member that either wears or does not wear Corrosion DR armor - and in both cases damage done was the same which suggests that DoT effects are not subject to Damage Reduction.