If you still have the original file try to find the MD5 sum of the file and compare it to the MD5 sum of the reconstitued file. If they match then the problem doesn't come from GNOME Split. If the MD5 sums are different that's a bug.
To merge the split files manually you can do "cat file1 file2 file3 > file". That should also work to merge files if you have used the generic algorithm.
If you still have the original file try to find the MD5 sum of the file and compare it to the MD5 sum of the reconstitued file. If they match then the problem doesn't come from GNOME Split. If the MD5 sums are different that's a bug.
To merge the split files manually you can do "cat file1 file2 file3 > file". That should also work to merge files if you have used the generic algorithm.