.Net SDK Method not supported error in BoxJWTAuth with .Net Core 3.1

I am just getting started with the .Net SDK, using .Net Core 3.1 (VS2019). Have set up the app, jwt, oath, and the boxJWT var is created/populated without issue. Am encountering an error on the 2nd line.

var boxJWT = new BoxJWTAuth(configFromFile);
var adminToken = boxJWT.AdminToken(); //Breaks here

The root error occurs on this line in the Box.V2 SDK:
BoxJWTAuth.cs
string assertion = tokenHandler.WriteToken(token);

Error Info:
NotSupportedException: Method not supported. System.Security.Cryptography.RSA.DecryptValue(byte[] rgb)

Box.V2.JWTAuth.BoxJWTAuth.ConstructJWTAssertion(string sub, string boxSubType, Nullable nowOverride) in BoxJWTAuth.cs
string assertion = tokenHandler.WriteToken(token);

Box.V2.JWTAuth.BoxJWTAuth.GetToken(string subType, string subId) in BoxJWTAuth.cs
var assertion = ConstructJWTAssertion(subId, subType);

Box.V2.JWTAuth.BoxJWTAuth.AdminToken() in BoxJWTAuth.cs
return this.GetToken(ENTERPRISE_SUB_TYPE, this.boxConfig.EnterpriseId);

BoxAPIdnc3.1_v1.Controllers.BoxTestController.GetAsync() in BoxTestController.cs
var token = sdk.AdminToken();

The Box.V2 build is .Net 4.5, so it should work? - MS doc - RSA.DecryptValue(Byte[]) Method

I've obviously done something wrong as at least one other user has it working.

Please advise - thanks in advance!