Why an array of variables cannot be used as a payload to pass along with the signal when async is set to true

The customer would like to use the API below and add a payload to pass variables along with the signal:

POST runtime/signals

However according to the documentation it is clearly stated that variables cannot be used in case {{async}} is set to {{true}}, as this will result in an error.

Could you please tell why? Is there a workaround to send a signal for asynchronous execution and also pass variables?

Refer: https://www.activiti.org/5.x/userguide/#_signal_event_received