Ok, I can explain. I apologize for any previous misunderstanding. When upgarding KBN from 7.17.X to 8.X, I wanted to preserve a few separate kibana instances(with its settings/saved_objects/visualizations/dashboards) by using kibana.index (in kibana.yml) instead of merging them to one common kibana(with default “kibana.index” .kibana) and separate with the only one officially supported option by Kibana, Spaces.
Thanks to Readonlyrest it works very well. Just reporting module(and report download) seems to be having some problems in my case 
My suspicion is that this Reporting issue is related to using different Kibana index names on different Kibana instances, combined with how Kibana/ES handle and coordinate reporting jobs.
Even if that’s not the root cause, the example I posted still stands: it shows the configuration required for Reporting to work reliably with multiple Kibana instances.
Thank you very much. I think that reporting in KBN 7.X with multitenant configuration could much rely on these parameters “xpack.task_manager.index & xpack.reporting.index“ that are not available in 8.X and users like me, should move to recommended Spaces