In an ideal WYSIWYG world the display during editing should show what the printed output would look like. That's not what happens. What you see depends on the dpi resolution stored in the original image file.
In my Scribus preferences (in Tools, Image Frame Properties), Scale Image to Frame Size and Keep Aspect Ratio are checked.
I have a bitmap image, black and white line art, 100 pixels wide. I scaled it in The GIMP to 50 dpi, and again to 400 dpi, without changing the actual 100 pixel width, and saved both versions as PNG. I created two identical image frames, about 1.4 inches wide, and imported one of these images into each.
The 50 dpi image, before scaling, would be 2 inches wide. It was placed in the frame scaled to fit the frame, and it looked sharp.
The 400 dpi image was placed in the frame at its expected scaled size, about 1/4 inch wide and anti-aliased. I right-clicked and chose Adjust Image to Frame, and the image was magnified without reloading the image, so the image looked blurred.
Going to File, Print Preview, errors were reported for both images, saying the resolution was below 144 dpi. That's true of the actual images as scaled in the document. I ignored the errors and both images looked perfectly sharp in the print preview.
I saved the document and reopened it, and the appearance did not change: the 50 dpi image looked sharp, and the 400 dpi image looked blurred. The Print Preview result was the same as before saving: errors, but sharp.
Live with it until somebody fixes it.