19c undo behaviour is different from earlier versions

It seems that 19c undo behaviour is different from earlier versions.

In 12.1 and earlier versions for fixed-size undo tablespace (autoextend disabled for all datafiles) Oracle would ignore parameter undo_retention and try to keep undo as long as possible. From the manual: “For fixed-size undo tablespaces, the system automatically tunes for the maximum possible undo retention period, based on undo tablespace size and usage history, and ignores UNDO_RETENTION unless retention guarantee is enabled for the tablespace.”

In 12c it was normal for undo tablespace to be 90 - 95% full. Actual undo retention was shown in v$undostat.tuned_undoretention and it varied depending on undo volume.