Checkout Screenshot 2021-09-08 at 16.51.09@2x.png in this repository.
In that screen you'll notice that the command logs incorrectly reports the intercepted request as modified even though it clearly wasn't as you can see the test spec in the cypress/integration/bug_spec.js.
I was able to trace down the issue to https://github.com/cypress-io/cypress/blob/e216ae2422706d4f46e6dfa68abe2d589c72cf96/packages/driver/src/cy/net-stubbing/events/before-request.ts#L289-L291.
For this specific case req and reqClone will be different, because req.body is a string whilst reqClone.body will be a parsed JSON body.
First, install the dependencies:
npm install
Then first run the example server and then cypress separately:
node server
./node_modules/.bin/cypress open