My P1P drastically fluctuates in temperature for both the bed and extruder when starting a print. Both temps will rise and drop randomly throughout the print and when I attempt to change the temp manually while it is printing it just continues to change on its own.
The bed physically can’t change temperature very quickly so if the readings indicate it does, the sensor is likely not reading correctly. If that goes for the nozzle temperature as well, maybe there is some deeper problem like a faulty analog ground somewhere. I guess you should just open a ticket with Bambu, including logs and pray that they respond semi quickly.