That's odd. Usually that happens when there is a debuff to be applied or something to that effect - but his A1 doesn't do anything special. It also has a lower multiplier than his A2 and A3.

Are you saying he uses his A1 despite the A2 and A3 being available?

He uses  his  A2  just  fine,  he  uses  it  everytime  when  it's  available.  He  doesn't  use  his  A3,  he  does  it  just  once  at  the  beginning.  I  guess  he  would  do  it  again  if  the  spider  gets  a  turn  and  the  debuff  counter  gets  down  to  one  but  well,  the  spider  doesn't  gets  a  turn  as  intended  xD

Yeah that's the weird part - his A3 has a higher damage multiplier than his A1, so even though the debuff is on the spider, he should still use it.

I'd say this is a bug.

I would say its not a bug. Since def down is already applied the AI saves it, this happens often with