12842 is not cleaned up due to very less TAT at MAS as it arrives MAS early morning as Chennai Mail. That's why it is not sent to yard for cleaning and is sent as 12842 to howrah after simple sweeping.
But at HWH, its well maintained and cleaned in yard. At Howrah, the rake of mail arrives in morning and is then sent as coromandal express in afternoon. 12841 is clean while originating from howrah but 12842 runs dirty.