It's probably Itch's download server being terrible. I've had problems with any game larger than about 200~400MB suddenly claiming it's fully downloaded despite clearly not being. The cure I've found is to try and download the file again (making sure it's in the same location and with the same name). It'll quickly run through what's already downloaded, and then start downloading the next block. You may have to do this a bunch of times, but at least it's not one of the 4GB games on the site.