When you import a PDF in ArtPro containing a very thin stroke, it is not imported. The object is visible but it doesn't contain a fill.
There is no solution for this.
The width of the path is too small (e.g. 0.003 points, or 0.5 internal ArtPro units), depending on how this value gets rounded you may or may not end up with an actual object. This is a known limitation in ArtPro, related to the internal resolution limit of 12000 dpi.
The workaround is to preflight the PDF looking for very small strokes and correct them.