How is a constant field throwing error with missing

Hi Experts,

we are facing a weird case in production PI 7.1:

The message is sending from PI to ECC via IDoc, and sometimes(not all times) we got error saying a specific node is missing from the IDoc adapter as below.

Actually this node is mapped with constant in Message Mapping, so it shouldn't be missing for the IDoc adapter to process.

As this IDoc is a standard IDoc, which is showing last update was some day in 1998.. and PI imported this IDoc in 2010, so I don't think it is related to metadata in PI(idx2), actually even tried to delete the metadata but still failing afterwards.

However, the most weird thing is the failed message can be reprocessed successfully without doing any thing-- why it was failed at the beginning?

Thanks for any idea shared!