listen guys, i hate it as much as the next ninjago fan, but Zane becoming more robotic in later seasons actually makes sense
So his power source is what made him 'human,' right? Whatever half-magical mysterious artifact Dr. Julien managed to find or make is what gave Zane emotions, and memories, and a heart. well as we all know, that was destroyed in season 3 when he died. And seeing as the show makes it clear that was a one-of-a-kind piece, Zane couldn't really have rebuilt himself with another.
So there's probably one of two options going on and both of them are kinda depressing:
Zane's been an ai this whole time (his essence uploaded into the digiverse code? sounds a lot like all those sci fi things of 'living forever' as ai yk? like the movie transcendence?)
which is slightly less sad, and makes more sense to me, Pixal split the half heart she had, so each of them had a little piece. But during Rebooted, they say Zane's unlimited power source is actually not so unlimited. So as time has gone on and Zane's been running off 1/4 what he was intended to, it's worn down and just lost life.
And yes yes ik it's really just a writing thing, but i like making in-universe explanations for every possible slip, so ha. sadness be upon ye.