As part of WL#11819: Support ACID Undo DDL Bug #28915743: UNDO TRUNCATION IN 8.0.13 IS NOT CRASH SAFE
Upstream is processing MLOG_DELTEF_FILE for undo.
which is not handled by the PXB and causing hang during backup
set global innodb_page_cleaner_disabled_debug=on;
CREATE UNDO TABLESPACE undo1 ADD DATAFILE ‘undo1.ibu’;
ALTER UNDO TABLESPACE innodb_undo_001 SET INACTIVE
Recovery log parsing was handing redo on ibd files.
and not on undo files which are changed in 8.0.21.
merge 8.0.21 with pxb. so pxb also handles redo on "inactive undo tablespace"