Per Dave Meltzer of F4WOnline (h/t Marc Middleton to WrestlingInc.com), Miller recently returned home to Australia for the birth of his child and decided to stay.
Meltzer added that the door remains open for the 33-year-old Miller to return to the company at some point.
Miller is one half of The Mighty with Shane Thorne, who remains under contract with WWE.
The Mighty signed a developmental deal with WWE in 2016 after an impressive run in Australia and Japan as The Mighty Don't Kneel.
They were renamed TM-61 and reached the finals of the Dusty Rhodes Tag Team Classic in 2016, but they were defeated by AOP.
Thorne missed nine months with a knee injury, and they returned to action as a team in January before turning heel.
They changed their name to The Mighty, but they were unable to gain much headway in NXT's stacked tag team division.
Rather than competing for the NXT Tag Team Championships, The Mighty primarily tussled with the likes of The Street Profits and Heavy Machinery.
With Miller reportedly out of the picture, it is unclear whether Thorne will be utilized as a singles wrestler or placed in a tag team with a different Superstar.
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