Jumping backward in versions can cause old incomplete files to not load, thus needing to resume from library window -> incomplete folder. The files would show zero %, but once the file actually starts to download again, it should show its previous download %.
If this is not the case, then I am not sure. As you have said, there is no other incomplete file with same or similar name.

AFAIK there would be no fix for your problem. Perhaps cancel the download. Search the topic again & see what ones you find. Perhaps making the correct choice 'might' continue it from where it was but I have some doubts.

The LW 5 beta/alpha handles some of the library information differently to 4.18 & earlier versions. This 'might' be the issue. I made a back up of my preferences, etc. before using the alpha version.