The ACA made rescission of health insurance more difficult for insurers, but it absolutely happens. There are people at insurers tasked with looking for fraudulent omissions in policy applications for high loss ratio individual policyholders to target for rescission.

This one is pre-ACA but the insurer focused on an "omission" to justify the rescission: https://www.cnn.com/2009/POLITICS/06/16/health.care.hearing/...