Not Really sure what to tell you at this point.
Perhaps upgrade to 8.15.2
I can't debug your logstash...
Compare Discover use breakdown event.dataset by 1 minute interval
Then Observability Overview for 30 Mins
They Looks the same for me, if they don't for you I am not sure what to tell you..
I would load some data from another method...
Then do the comparison if that works then I would look at your migration logstash pipelines.