I simply cannot reproduce it. I just selected 356 files adding chunks of a about 80 files at a time and the display of "cover varies" stayed.
So it would be interesting to either see the 140th file and what is different about that or to learn more about your environment:
do you have the files locally?
Do other programs access the files at the same time (so that reading the tags gets delayed)?
Which types of files are these?
Did you check the files for integrity with the known tools?