Just swapped my old T2100 G4 box for a UHD G5 box and run into this issue. Note that this is not down to CEC as I have tried turning CEC off on my TV which makes no difference.
I need to explain that the G5 box is actually connected to the TV through an HDMI switch and it is actually when the switch is turned on that the G5 box turns itself on. This suggests to me that it is detecting a control voltage on the HDMI connection with the switch (even if the switch if switched to another input).
This to me seems to be a bug in the G5 as it did not happen with the old G4. As a workaround I have changed my Harmony remote control to leave the switch turned on all the time. A longer term solution should be when I replace my ancient and failing AVR with an new one and no longer need to use the switch for HDMI switching but problem sourcing the AVR model I want at the moment due to the ongoing semiconductor shortage.
One other workaround I can try is to connect the G5 box directly to the TV swapping connections with my little used Blu-ray player and bypassing the HDMI switch.
This should identify if the switch is also partially at fault by sending out control signals on all ports when it is turned on or another connected device is turned on or off.
Must remember to move the CEC-killer adapter too (did I mention I also use one of those?)