You can modify the content of the request directly in the JSON window/panel (the JSON in the right panel).
It is likely because the flow execution allows variable to be of any type: string, number, boolean, ...
And that's not something which can be represented in current swagger specifications (type any).
This is necessary to enable if you're trying to send a request that doesn't match the schema in the request builder wizard. The wizard "fixes" the JSON document for you to ensure it adheres with the schema. Pro mode turns that feature off and lets you type and send whatever you want, even if it's incorrect per the documentation.
The way the schema formats the inputs in the API explorer doesn't work you need to use the pro mode as mentioned above, here are some screenshots of one i have set up.