The isHittable property of XCUIElement is a total disaster. It doesn't behave as the documentation indicates, and worse than that it randomly kills tests with this sort of failure when it can't figure out whether an element is hittable or not:
Failed to determine hittability of "storeListSearchField" SearchField: Activation point invalid and no suggested hit points based on element frame
This is a longstanding issue that hasn't received any notable attention. The behavior of this property is totally wrong even in its response to an inability to determine the "activation point". If this point can't be ascertained, the property should just return false, not kill the test with an unsolicited assertion failure.
I just went thru the arduous task of removing all reliance on the isHittable property from our UI automation framework and tests, replacing it with our own "scroll margin" implementation. This is far from ideal, because this implementation doesn't have access to the element layering information available to the display rendering engine. However, this is a definite improvement over the erratic behavior and random failures of isHittable.