I've had the Yema Kickstarter GMT bronze since release. It went back to the factory twice due to extreme variance in accuracy.
Since second return it has been +10 to +15 per day. A bit high for my liking but I lived with it.
I went to Turkey last week and noticed it was suddenly very accurate at 35°c. I read that temperature changes can affect accuracy. Anyway, came back to wet UK and it is still pretty accurate. Fingers crossed but any guesses what happened? Was the heat just a coincidence?
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My best guess would be the temperature change affected the oils in the Watch in a way where you have noticed the change.
It may be magnetized. try to demagnetized it.