If I change your sample it does not work:
remove the radio buttons and add
td.Cancelable = true;
var tdb = new TaskDialogButton("tdbBtn1", "Custom Button 1"); // TaskDialogCommandLink does not work, too
tcb.Click += (s, d) => td.Close(TaskDialogResult.CustomButtonClicked);
the only way to close this TaskDialog is using the X.
But thanks, you pointed me in the right direction. The culprit is that Rhino is shipping it’s own ApiCodePack dll. It looks like an old version. It’s definitely missing the fix https://github.com/aybe/Windows-API-Code-Pack-1.1/commit/5de444bd2f51ea935e244b7b8ce405a2cdc809a1 as I can see in the decompiler.
How are the chances for the fix in a Service Release?