When a Cerbo, or some other GX device or DMC, is connected to the VE.Bus, then the physical switch is ignored.
Did you use both the Cerbo and the Mk3 at the same time? The Mk3 can emulate a DigitalMulticontrol, which would disable the Cerbo ability to remotely turn on/off the Multiplus. Not the exact same situation here, but could be related. To return to full functionality once the DMC was disconnected, you needed to rediscover the VE.Bus device on the Cerbo.
From watching the video, i suspect that the MP at first goes into standby for a few seconds, then fully powers down. Thats why you can enable it again for a short period. You can prevent it from fully powering down by removing Pin 7 of the VE.Bus cable, the MP will then listen to the physical switch again, but to me thats always been a patch for something that is an underlying issue somewhere. It will also use more power from the battery when permanently in standby compared to fully powered down.
AFAIK theres no final solution to this
If you are able, you can try to measure whether PIns 6 and 7 get pulled to GND once the Cerbo wants to switch on the MP. Pin 6 is standby, if its freefloating the MP will fully power down, if its pulled to GND it will stay in standby. Pin 7 is remote control connected, if its freefloating then theres no remote control, if its pulled to GND then there is.
The expected sequence is Pin 7 is always pulled to GND, as long as the Cerbo is connected and switched on, once you press “On” on the screen, Pin 6 should get pulled to GND, then the MP will change into standby, then the On command will be sent over the data lines from the Cerbo to the MP, once the MP has powered on, Pin 6 can be freefloating again.
Maybe your Cerbo has an issue with pulling Pin 6 to GND, or your cable has a fault on that specific conductor, or the MP itself