Can You check "
In portion below and try to see if You have the same issue.
- Applied the latest patches to OEM environment and ASH Analytics page and SQL monitoring are blank on version 12.2
- Document 2562009.1 did not solve the issue
This just talks about the package installation to get the ASH analytics page displayed in 13.3.
But might not help actual query Fabien had.
I would suggest you to check the following document.
EM13c : Pluggable Database Administration (Doc ID 2628573.1)
This document helps you to understand how to create an admin which will have privileges specific to a PDB targets.
Thanks for pointing that out.