Under certain conditions - for example, with a large payload or when buffer reuse is forced by client behavior - HttpServerExchange.getRequestReceiver().receiveFullBytes(...) performs the actual async buffer read and, importantly, invokes the read-body callback on the I/O thread which prematurely unlocks the dispatch(...), so the handler returns before the request is fully processed.

public void handle(HttpServerExchange exchange) throws Exception {
    exchange.dispatch(() -> {
        // ...
        exchange.getRequestReceiver().receiveFullBytes((ex, messageBytes) -> {
            // ...async work here, which needs to
            // be dispatched again since execution
            // might be in the I/O thread.
            // This is not obvious, because it's
            // not documented and not consistently reproducible.
        });
    });
} 

I’ve put it as Minor, since I’m not sure whether this is a bug or undocumented behavior – but it was painful to figure out, so at the very least it would be better if this was clearly documented.