MEP 401(k) Plan-Does account balance transfer with eEE when changing ER?

[fynn6%7]

Company A established a 401(k) plan in 1999. In 2008, the owners of company A purchased company B and we had a controlled group situation. Late in 2016, ownership changed and there was no longer a controlled group but the plan has become a multiple employer plan. Employee Z contributed to the plan from 1999 to 2005.  In 2016, he became a 10% owner of company B. He is currently not working for company A. He has an account balance attributable to contributions made while working for company A. He is not currently making contributions.

What happens to his existing account? Should it be accounted for under the assets of company A? Or, is it now counted as a company B asset? Plan B would become top heavy if the assets move to that plan.