The SinginManager has many methods that are dependent on an injected HTTP context. These methods may not function as expected in Blazor Server.
Avoid IHttpContextAccessor/HttpContext in Razor components
As far as I know, the UserManger does not have dependencies on the HTTP Context and should be okay.
The source code is on GitHub.
https://github.com/dotnet/aspnetcore/blob/main/src/Identity/Core/src/SignInManager.cs
https://github.com/aspnet/AspNetIdentity/blob/main/src/Microsoft.AspNet.Identity.Core/UserManager.cs