Where does the interrupt handler return to?
What happens internally inside the processor could go either way depending on how the designers chose to implement it.  Either you are returning to the address that was interrupted, but it immediately gets interrupted again, in which case the next instruction that executes is the first instruction of the next handler, or you are jumping directory to the first instruction of the next handler.

In either case: the first instruction of the next handler will be executed before the instruction that was originally interrupted.  Anything else would be a priority inversion bug.