That looks like a bug, I'll investigate it. BTW, were there any download errors during the download ? Could be that you got banned during the download for downloading too much tiles (some tileservers do that) and some tiles didn't get downloaded. Also try to do the batch download with "redownload ON", which can be set from the "edit" menu. This makes sure that all tiles are downloaded for the given area, even if there are already some (corrupted ?) tiles present. Might help.