Fugured it out myself.

the tv has three modes for remote power on:

1. off
2. through network
3. through application (adb)

if you choose the third option, the tv periodly "pre power on" itself so that user can send adb command to it to actually turn on the screen, which causes my HDMI cable periodly receiving signals of new device.

(still not sure why the tv can't be always "pre powered on")

But if you choose the second option, everything is fine.