This happens pretty often. When looking at artist's albums I sometimes see songs that I've added to my library as not added, however they are in my library! What I do then to keep it clean is to remove all songs I have in my library and then add them once again. That doesn't make sense.
And It's not about single/full album releases after merge. This sometimes happens without any reason.
And even if this shouldn't be happening. Please hire devs and start fix your bugs it's ridicilous
Thanks for getting back to us with this information.
There are multiple versions of the same album or song that are added by artists or right holders. This means that if you save a song from one album that has other versions (i.e. deluxe version), only one will show as saved, and other versions will also have the option to save them. This is why you can have the same song saved multiple times.
If anything else comes up, don't hesitate to open a new thread.