Your error doesn't appear to be related to the CSP. The CSP error you see in the console is expected, and it's unrelated to the experimental support that shipped in 8.3 (which is disabled by default).
But problem exist for other people too, even when direct connect to Kibana - without Nginx between.
On 8.2 it works, on 8.3 didn't
EDIT:
After testing, I have same CSP errors on older versions and everything work correct. Just because of unknow reasons after update to 8.3 kibana stoped working. System services works fine, no error. I can direct connect to ES and it work ok.
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