Seems my CoreBook X (2021, i5-8259U) has just developed the same issue, after being used for about 6 months. The model is different but the keyboard flex cable is the same.
I was working on it, not doing anything particular, not even typing. It suddenly powered itself off, then on again, then off after 3 seconds, and kept doing so until I disconnected the battery. It took a while for me to realize this was actually the culprit.
It’s just the power button, all the other keys are working. Seems it’s connected separately. I didn’t check the trick with holding another key since I only looked at the forum after working around the issue so that the laptop can still be used, temporarily.
I isolated pins 27 and 28 on the keyboard flex cable, plugged in the battery, briefly shorted pins 27 and 28 on the connector, and then connected the flex cable. This way I can use it for the time being but I have no easy way to power it back on if it turns itself off. So I have to think of another solution.
I’m traveling abroad now, so I don’t have another laptop, nor any tools, or a multimeter. It’s quite an annoyance to run into this issue.