Logged into a PDB as SYS. Highlight a table or view, in this case UNIFIED_AUDIT_TRAIL and hit F4 and get the pop-up error "Synonym PUBLIC.UNIFIED_AUDIT_TRAIL does not exist. This also occurs when running "desc UNIFIED_AUDIT_TRAIL;" from an editor window.
However, Oracle has no trouble running "select count(*) from unified_audit_trail;" without qualifying the owner (AUDSYS) with the object name.
This was a problem with the 16.3 beta's as well, however I do remember it working in previous releases.