Is leaving for an employer’s non-US affiliate a severance-from-employment?

XYZ US and XYZ UK are commonly controlled business organizations.

XYZ US maintains a 401(k) plan. XYZ UK is not a participating employer under that plan.

Martha ends her employment with XYZ US on June 30, and becomes XYZ UK’s employee on July 1.

If employment by a business organization commonly controlled with the 401(k) plan’s sponsor otherwise would mean a change is not a severance-from-employment, is there anything that varies such a rule if the next employing organization is outside the USA?

I don't think so. Furthermore, I don't think that person is statutorily excludable from coverage and discrimination testing (assuming Martha is US citizen) as none of the listed exclusions apply. She simply goes from an eligible class of employee to non-eligible class of employee, and still earns vesting service for employment with XYZ UK. 

Thank you for your quick help. And thank you for mentioning some other points.

It seems Martha, still a 20-something, might need to maintain distinct retirement-savings accumulations under two or more separate plans (XYZ has commonly controlled organizations in several nations) for about three decades or more.

Yes, the lovely nuances of international work!