With Health Insurance, a policy is always contestable.
That being said, each company handles pre-existing conditions differently. It also depends if you have a group policy, individual policy, and if you had credible coverage before.
With some, you may not be eligible, with others, they may excluded, or rider that conditions. There are some that will have you wait 12 months before it is covered and some that may cover that condition right away.
That's why honesty is truly the best policy.
Your thinking of Life insurance, where the contestable period is 2 years, after which the policy can not be rescinded other than for non-payment.