In patch 2.1.0 there was a fix to the Nemesis Bracers:
- Fixed an issue that caused Nemesis Bracers to only spawn Fallen Champions when used inside Nephalem Rifts
That fix was never ported to Greater Rifts and only Fallen Champions are spawned by pylons. Most likely, the fix was forgotten to be ported when GRs were introduced in that same patch.
Note that this applies only to the actual champion spawned, not its minions which remain random.