DB/DC Aggregate Testing issue

A controlled group of companies, C, sponsors a 401(k) plan. Unbeknownst to C, one of its subsidiaries, N, in order to win a contract signed on as a participating employer in a multi-employer pension plan H.  

H has a frozen DB plan and a 401(k) plan, 12 of N's employees are eligible under both C's 401(k) and H's pension. H's DB plan could not pass testing, so using SECURE Act relief, they  aggregated H into a DB/DC plan and then demanded data on C's entire control group of 4,000 employees - which C gave them. The H plan then passed 410(b) and 401(a)(4) testing.

Of course, C has been performing its own independent ADP/ACP testing for years, not even being aware of H until the recent demand for control group data. H's actuaries are now recommending that C go back in time and re-do their ADP/ACP testing to aggregate H's 401(k) data. H's actuaries admit that the SECURE Act relief that allowed H to aggregate its DB plan with its DC plan, and thereby pull in C, is silent on how H's decision to aggregate would affect C.

So what say you experts? Does C have to re-do its historical ADP/ACP testing to add H's data to the mix?