If we want to use the alias, is needed to pass in one (or more) processInstanceIds. The logic to retrieve the container id is based on the content of the ProcessInstanceLog table, using the processInstanceId:
https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-services/kie-server-services-jbpm/src/main/java/org/kie/server/services/jbpm/ProcessServiceBase.java#L212

https://github.com/kiegroup/droolsjbpm-integration/blob/master/kie-server-parent/kie-server-services/kie-server-services-jbpm/src/main/java/org/kie/server/services/jbpm/locator/ByProcessInstanceIdContainerLocator.java#L51

Customer In this particular scenario, are having the signal at container level. So when we trigger the signal, we will be providing only the container - not any processinstance ids. The problem is that, the API is not working with alias

We can use one of these API calls:
http://${HOST}:${PORT}/kie-server/services/rest/server/containers/${CONTAINER_ALIAS}/processes/instances/signal/${SIGNAL_NAME}?instanceId=${PROCIDS} // here PROCIDS can be a single id, or a comma separated list of ids

http://${HOST}:${PORT}/kie-server/services/rest/server/containers/${CONTAINER_ALIAS}/processes/instances/${PROCID}/signal/${SIGNAL_NAME} // to signal a specific PROCID

Its necessary to send the signal using container alias without send instances id