Have not seen this. I just imported an album for which a 1400x1400 image was in the same folder. It's still at the same resolution after import.

If you point your browser at:

http://<core IP address>/debug/albums then look for the problem album and click on the [dir] link, do you see the cover in the original size there?

For example, I see:

cover.img02resize170.jpg File
cover.img02resize256.dds File
cover.img02resize256.jpg File
cover.img02resize512.dds File
cover.img02resize512.jpg File
cover.jpg File