This issue only reproduces under quite special conditions.
I have an object with 2 properties of type DB Tree List and properties. What I noticed is that, after saving the document, one of the db tree list property (always the same) was receiving the values of the other property, but only in the cache, nothing was saved in the database.
The issue reproduced all the time, on all types of save: document save, copying the document, etc.
The issue is that a document that is broken in the cache
1/ displays bad information,
2/ when it's saved it's saved the bad data can be saved in the document,
3/ when it's copied copied is copied with the bad values in the new document.
However, if I change the document and change the value of the field that got broken, it was not always reproducing for all values of the field.
Further investigations show that the issue is reproducing for all the list properties for which the string representation of their value is longer than 255 characters (if the list is a multiple select, the sum of all values must be longer than 255).