Images cropped with Briss do not show when inserted in LibreOffice Impress

[rgudwin]

For some unknown reason, when I take crops from a PDF with briss and insert them into LibreOffice Impress, the final PDF generated by LibreOffice, does not show the cropped image. Other PDFs inserted just like these do not present this problem.
Do you have any clue what could be happening ? This strange effect of not showing up happens just with images generated by briss from an original metafile PDF. If I take the cropped PDF and convert it to a bitmap, it shows up, but then the metafile resolution is lost, due to the conversion to bitmap. It is really a strange effect.