From my debugging and googling, the latest version of Tabbable that is 5.3.0 (and onwards) needs some changes to the Modal/Wizard components in Patternfly4 to be able to support tests.

Specifically this: https://github.com/focus-trap/tabbable#testing-in-jsdom

Modal currently has the prop disableFocusTrap that can be set to false and that lets us work around the issue without ever activating the trap. I spoke to folks on PF4 slack and they lock the version of FocusTrap but Focustrap brings in the latest 5.x.x tabbable causing the test failures.

There's an open PF4 issue around this: https://github.com/patternfly/patternfly-react/issues/7288