Provider directories are maintained by carriers and are frequently out of date. A physician may appear in a directory months after leaving a network, or may participate in some of a carrier's plans but not the specific plan you are considering.
That last distinction is the one that catches people. Carriers sell multiple plans with different networks. A doctor who is in-network for a carrier's employer group plan may be out-of-network for its individual Marketplace plans. Searching the carrier's general directory rather than the specific plan's directory produces confident, wrong answers.
The reliable method has two steps. Check the plan-specific directory, then call the provider's billing office and ask whether they are contracted with that exact plan for the upcoming plan year. Billing offices know this in a way front-desk staff often do not.