I changed the games canon so that it is Solas who teaches Siryn rift magic.
Because Siryn thinks Your Trainer is bonkers and she does not trust this woman to teach her magic. And Solas knows the same magic while remaining very much sane, so he’s the more logical and natural choice. Also, she thinks he knows more on the subject than this muddleheaded woman.
(Which he most certainly does. For all we know, Solas could have invented/discovered rift magic.)