Apologies @Wissu, this issue currently affects all owners of the Necromancer Cosmetic Upgrade DLC, with specifically the Soulstealer Staff illusions being locked/missing as you describe. Please rest assured we’re on the case and this should be resolved soon hopefully!
This thread was added on March 08, 2024, with posts from FatsharkLev.
UPDATE
We are aware of an issue introduced with the 6th Anniversary update which has caused weapon illusions for Necromancer’s Soulstealer Staff to become locked.
Please be assured your weapon illusions are still owned by your account and we’re working to resolve this in an upcoming hotfix.
Apologies for the inconvenience in the meantime.
Existing and new bug reports will be merged here to keep things tidy 
This issue has been resolved as part of Hotfix 5.3.3