Two same AJAX calls – one is working, other doesn’t

They’re not the same though. Look at the form data on each one. The second one, the one that doesn’t work, has _test appended to the parameter names and the action value.

The reason you’re getting a 400 error is because you’re not sending a valid action. The action parameter is how WordPress determines which callback to use to handle the request. If you don’t send action with a valid value, no callback exists to handle the request and it returns a 400 error.

You need to make sure the parameter is called action, not action_test, and the value is the proper name you’re using in PHP. In your case that appears to be return_reason, not return_reason_test.

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