@Luca_Lattarini did you check this - https://experienceleague.adobe.com/docs/experience-platform/identity/troubleshooting-guide.html%3Fla...
Identity Service only links identities when single records present two or more identity values. This error message occurs once for each ingested batch, and displays the number of records where only one identity could be found and resulted in no change to the identity graph.
For Id Graph to work right it needs min of 2 ids - if a record has only one id that's where its skipping to be shown on id graph. That doesn't mean record is not ingested in UPS - its just not shown in Id graph.
In AEP remember we have different storage, this process failed to update the Id graph since it has only one Id.
- Data Lake
- Unified Profile Store (UPS)
- Identity Graph
- Edge storage