That sounds familiar; I first ran into this issue over a year ago (I drive a Kia EV6).
Sometimes things would go fine for days after selecting ‘stop charging’, sometimes for just an hour, and other times it would resume immediately.
It’s not the Homey app’s fault I think; I had the same problem with the Kia app itself. The restart is triggered by the car itself, or by a combination of the car and the charging station (I have a Zaptec).
Even when I paused charging via Zaptec, it would sometimes resume on its own.
I never managed to figure out exactly what causes it, but I’ve heard others mention the same issue. I raised it with both Zaptec and Kia, but neither really did anything about it.
What I do now to stop charging reliably is set the charging station’s current to 0A. The station does this automatically when I use its scheduling feature. And this works. I know that some ‘smart charging providers’ also use this trick.
Lowering the car’s charge limit works too, but in my case, I can only lower it to 50%.
Those Korean idiots just don’t get how important smart home technology is for car manufacturers too. As for me, I won’t be buying another Kia or Hyundai.