When you do Descartes > Rasterize and select the Stamp command and select vector text it changes text to green and you click Accept not all of the vector text data is converted to raster text. It does not happen on all text in the DGN file.
Defect 492486 has been filed for the issue where the Stamp command is not rasterizing the complete text data.
A workaround is to select Edit Text and highlight the text and Accept. The Stamp command can then be used to convert the text data to raster.
Video shows workaround for rasterizing vector text data.