Contributing to Wholly Owned Subsidiary

[khn]

As of 10/1, Company A became a wholly owned subsidiary of Company B. Company A's employees moved to Company B's payroll, but continue to work for Company A. 

Company B wants to keep allowing Company A's participants to contribute to their plan until the plans are merged in 2025 but their TPA is saying if they are being paid by Company B they are Company B employees and can no longer contribute to Company A's plan. is that correct?

In past acquisitions, Company B has moved employees to their payroll and continued to remit contributions to the acquired entity's plan until the merger date.  Why would they not be able to do that again?