The C# BSON creates a ObjectId value with a call to System.Diagnostics.Process.GetCurrentProcess().Id. The Blazor Webassembly framework does not support calls to Process.GetCurrentProcess() (see here and here)
Welcome to the MongoDB Community Forums. Thank you for filing CSHARP-4551 as a feature request. This is a reasonable request and we are discussing when we can schedule this work. Please follow the linked ticket for updates.