Hi Paolo,
Greetings from Syncfusion support,
We have checked the shared sample. We have internally considered the reported issue with the SfToast, and it is resolved with the Nuget package version 18.3.53. With the shared sample you are using the version 18.3.52, which is the cause of the reported issue. Can you update the Nuget package to latest version, and check the reported issue from your end ?
Please get back to us if you face any difficulties,
Regards,
Indrajith