Thanks for answering my questions. Knowing that, we should avoid making the Document ID numeric, but use a seperate numeric field instead in our project.

Besides that, I was not able to create a simple test case that reproduces the problem.
One other thing I figured out is that it is happening when the class in question gets validated via an @IndexedEmbedded annotation inside another class.
If I remove this @IndexedEmbedded annotation, everything is OK again.
But still, I cannot create a test case... there must be something more to it.
I'm not sure if this is worth to investigate further since the plans are to forbid customizing the document ID in future releases.