This seems to be the same problem which occurred with SQL2017 CU21 and is now fixed:
This problem is consistent with msdb. When I run:
DBCC CHECKDB ([msdb]) WITH NO_INFOMSGS, ALL_ERRORMSGS, DATA_PURITY, EXTENDED_LOGICAL_CHECKS;
error 2706 is always returned even though no errors are reported.
If EXTENDED_LOGICAL_CHECKS is removed CHECKDB runs fine.
Like the SQL2017 CU21 problem I suspect there is nothing actually wrong with msdb. ie This would not stop me from applying SP3 to production.