Can't banish devils?

[Forblaze]

I'm in the fight with Yurgir and he failed his saving throw against banishment. I immediately misty stepped out of the fight. I waited a few real time minutes and then went back, but he's still there. As a devil, should he not be permanently banished back to hell?

[Solomon Kane]

Read Spell Description, the Spell does not work like its Pen and Paper version does.