If I understand correctly, the popular interpretation of how a pit fiend's at-will trap the soul SLA works is that it doesn't require a gem in which to imprison the victims.
If I follow that interpretation correctly, if the pit fiend zaps someone with the spell and they fail the save, they disappear into hammerspace or wherever and the pit fiend can later extract the victim from hammerspace at its leisure.
Let's say a perfect storm happens - the pit fiend casts trap the soul on someone with spell turning (perhaps courtesy of an intelligent ring with a readied action). The spell rebounds, and against the odds the pit fiend beats its own SR and fails the will save.
What the heck happens to the pit fiend? Does it just wink out of existence?
How would the pit fiend get freed if there's no gem for anyone to find?
Disclaimer: I'd require the pit fiend carry some gems around, because cramming someone into a gem is actually part of the spell effect. And so it'd wind up in an appropriate gem. But by RAW the pit fiend doesn't need a material component (the gem in question) because it's an SLA. So I'm curious how the rest of y'all would handle this.
I have it form a soul gem.
RAW and RAI it does not need a gem, just like the shadow demon does not need another receptable for its magic jar ability, and it is incorporal so it can't even hold any physical item.
To answer the main question I don't think there is a RAW answer. From a flavor point of view I always saw it as the Pit Fiend stealing your soul and owning it. The only way to get it back is to kill the Pit Fiend or get him to agree to release it.
How I would rule(ok I am finally going to give an answer): Due to my opinion on the flavor and the lack of a gem I would say the Pit Fiend would not be able to trap its own soul. I have no idea what a developer would say in an FAQ.
The pit fiend only needs to roll a 13 to beat his own SR with his own Trap the Soul. This probably happens all the time in Hell.
You automatically bypass your own SR per the rules. They didn't intend for your own "bad" spells to be cast on you, but that is how SR works.
edit:An 11 is needed, not a 13 which would still be over a 50% chance for the spell to fail, if the Pit Fiend was not on autopass.