I have ack enabled for a HEC input. I can successfully send data into splunk with guid #1. With the same curl but a different guid #2 the data is not in splunk but the response is Success and the ack Id request also returns true.
I looked in splunkd.log and I see this:
08-09-2022 17:10:04.755 -0700 ERROR JsonLineBreaker [49865490 parsing] - JSON StreamId:0 had parsing error:Unexpected character while looking for value: 'c' - data_source="http:dev", data_host="host:8088", data_sourcetype="sourcetype_name"