Sudden heated bed temperature deviation (>10°C colder than set) on MK3S  

Sudden heated bed temperature deviation (>10°C colder than set) on MK3S

Hi together,

since last week PLA prints just didn't work anymore, they will not stick to the PEI bed. Before it was never an issue, not PETG nor PLA.
Nozzle distance to the bed and speed and all that stuff hasn't changed and was double-checked.

Further trouble shooting showed, that the temperature of the bed was set to 60°C, but in reality (measured with a IR Probe) was only 52°C,
on the outer region 5cm off the bed's corner inwards it was even less (48°C).

I did a test and set the temperature of the bed to 70°C and let it sit idle and measured after 15 minutes, it was only 59.4°C in the middle of the bed.

I did the same test on my Ender-3 and set the bed to 60°C and measured exactly 60.0°C. So the IR Probe is OK.

What could be the cause of this? I'm sure it worked in the beginning, but now it shows more on the LCD than it really has.
I wonder if the temperature probe on the bed is loose somehow, but this would set the bed's temperature higher, not lower?
My workaround now is to set the bed temperature 10°C higher, but this can't be a real solution.

Do you see the same behaviour? What temperatures do you guys measure when setting the print bed to e.g. 60°C?

Thanks in advance,
Wolfgang