Alright folks, this is the answer from API support:
This is a known issue and we already have in our book the following SPR –
SPR 1064041 - CopyTree Method (IEdmVault19) with the mbIncludeDrawings does not include the drawing
I do not comprehend an workaround for this at the moment, atleast not from the PDM API side.
You will be auto notified when this resolved.