When you have syndromes with a huge number of symptoms, including extremely common conditions, and the symptoms are largely untestable, relying on patient self-reporting, then you're going to catch a lot of people with these symptoms.

This is the exact same thing with Long Covid, or Gulf War Syndrome.

The most interesting thing about Havana Syndrome is that it apparently only affects English-speaking diplomats in American and Canadian embassies. This is probably because they're familiar with the syndrome from news reports.