Line showing up beneath imported graphic
I am importing images in a PDF format by reference (Esc+fii) into my FrameMaker documents. On the initial import, everything is fine, but as soon as I wrap the anchored frame around the graphic, a faint line shows up at the bottom of the frame. If it were only visible in Farme I could tolerate it, but it also shows up in the PDF output when I publish, where it is unnecessary, distracting, and just plain wrong. There's no object style applied to the anchored frame.
Editing to insert: This also happens when inserting PNG images by pasting, so it's not specific to either importing by reference or to PDF graphics.
I've tried changing different properties for the anchored frame, and so far nothing works. I tried changing the Stroke Line width to 0, but the line stays (and the width changes to 0.015 on its own). I've tried changing the Pen Pattern to one of the ones that appears to be blank, but the line stays.
How do I get rid of this line? Why is it even showing up?
Windows 10
FrameMaker 15.0.4.751
(As an aside, I'm not adding a topic tag because there doesn't seem to be one appropriate to issues with graphics.)

