Sounds like a bug where the app is confused. Perhaps one of the machines was not connected to iCloud Photos, and then later was. The app might make two sets of albums, or it could be something else.
I would start with one device and try rating an image and seeing which set of albums it uses. Assuming you are connected over iCloud, then it should show up in the album on the other device.Then you know which one is the real one.
Then quit Nitro.
Use Photos (that’s easier) to move the images from the disconnected albums to the iCloud connected albums. That will update everything on both sides. Then you can delete the unnecessary folder (which will get rid of the albums too).
Then open Nitro and it should find the albums and be okay. If that doesn’t work, we will have to try something else.