Last Known Successful DBCC CHECKDB
Community wiki answer:
From CHECKDB From Every Angle: When did DBCC CHECKDB last run successfully? by Paul Randal:

One of the new features we put into SQL Server 2005 was storing the last time that DBCC CHECKDB completed successfully (called the last-known good time). It’s the time that the DBCC CHECKDB started, not completed.
What does successfully mean? This is the catch – if DBCC CHECKDB runs to completion then it considers that a successful run – EVEN if it found some corruptions. However, the last-known good time is ONLY updated if there were NO corruptions found. Slightly confusing I know.

Does seem somewhat contradictory, but I agree it sounds like this is not expected behaviour for that field to be updated if errors were reported.
On the other hand, your examples are quite minor issues, easily fixed by running DBCC UPDATEUSAGE so maybe SQL Server regards that as a successful run.