When the karma seal is activated there are marking on the right side of his body. I don't see how they could overlook that. It has to have been on purpose. That leads me to a few different possible explanations of what caused his byakugan to activate.
The first thing I thought was that his body was reacting to Naruto and Kurama's fusion technique.
Then there is the possibility that his Otsutsukification is complete and Momoshiki has completely taken him over. Jigen was controlled by Ishiki even when his karma seal wasn't active.
Or his byakugan awakened on its own. Don't forget he's hyuga clan.
Why didn't Kawaki have a byakugan when he transformed? Jigen didn't have one until he became Ishiki. What makes Boruto different?
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