I don't think it's wrong to call it integer overflow, as that covers wrap around in either direction. I don't think there is any mention of "integer underflow" in any IEEE standard.

There is however mention of arithmetic underflow, but that relates to a precision issue with floating point numbers, and not negative wrap around. (See IEEE 754).

It's still understood what one might mean by integer underflow. But if anything, I'd say it's a further specification of a type of integer overflow, and certainly not a necessary distinction.