IOC is different from the other interrupts in that there are individual flags in the IOCxF registers that you have to clear manually.

The DT-INTs routine doesn't know which of these you're using, so you have to do it yourself.
It will try and clear the IOCIF bit in PIR0, but that bit is read-only so trying to clear that doesn't have any effect.

I don't think the interrupt priority setting has anything to do with this.