Ok I think something is going on with the upload itself doing something to the extended attributes of the .app file. When you download the file can you run xattr -cr /path/to/application.app against the .app file. I found this posted on https://itch.io/t/506614/is-damaged-and-cannot-be-opened-error-on-macos for reference. From the man page for xattr this is what’s going on
In the fifth form, with the -c option (``clear’’), causes all attributes (including their associated values), to be removed. -r If a file argument is a directory, act as if the entire contents of the directory recursively were also specified (so that every file in the direc-tory tree is acted upon).
So it’s going to clear the extended attributes of the .app file.