Sorry for not replying previously. I recently upgraded to scribus 1.4.7, and I am still seeing the same issue. I am on Antergos (Arch Linux derivative, kernel 4.17.4) and I install scribus via the the Arch community repo (I tried installing 1.5.4 from the user repos but it would not build correctly).
When I do File --> Import --> Vector, the exported PDF looks great, but when I try to link a PDF file via the Insert Image box, I see the error I reported previously and the output is pixelated cannot be edited as a vector in Inkscape. I have tried with different PDF and EPS files and the result is always the same.