It's not really a bug, the app is apparently not intended to support redirect links to files hosted off-site, as responded to in this git issue. In the upcoming version 25, the downloads folder (where mid-extraction compressed files and installers tend to be stored by the app) should properly empty out failed or cancelled downloads, including empty files made by redirect links mildly confusing butler.
hebilee could decide to eventually upload the demos to itch and that should let the app properly install them.