Headers with underscores (e.g. key_login) are accessible in dev but missing in build mode

Environment

Nitro version: 3.0.260311-beta
Node version: v24.5.0
Package manager: yarn

Reproduction

export default defineEventHandler((event) => {
const headers = getHeaders(event)

console.log('headers:', headers)

return {
key_login: headers['key_login']
}
})

Describe the bug

Headers containing underscores (e.g. key_login) behave inconsistently between dev and build modes.

In dev mode, the header can be read normally.

However, after building the project and running the production server, the same header cannot be retrieved from the headers object.

This leads to different runtime behavior between development and production environments.

Additional context

The issue appears to only affect headers containing underscores (_)
Headers without underscores appear to work normally
The same request and handler code are used in both dev and build modes

Logs

Start by reproducing the provided defineEventHandler and getHeaders example in both dev and build modes, using headers with and without underscores. Compare the resulting headers objects and trace the production build's request-header handling. Done means underscore-containing headers such as key_login are accessible consistently in both modes, with regression coverage for the difference.