There was a major change in IdentityServer4 version v4 they are no longer setting the aud claim by default.

Probably you followed an old article, like this for example:

https://medium.com/@marcodesanctis2/securing-blazor-webassembly-with-identity-server-4-ee44aa1687ef

Which is using IS4 v3

But if you check the configuration section of the oficcial documentation it says you need to disable the aud Claim:

https://identityserver4.readthedocs.io/en/latest/quickstarts/1_client_credentials.html#configuration

{
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();

        services.AddAuthentication("Bearer")
            .AddJwtBearer("Bearer", options =>
            {
                options.Authority = "https://localhost:5001";

                options.TokenValidationParameters = new TokenValidationParameters
                {
                    ValidateAudience = false
                };
            });
    }

    public void Configure(IApplicationBuilder app)
    {
        app.UseRouting();

        app.UseAuthentication();
        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}