Prusa i3 MK2 Heated Bed Thermal Runaway Problems
The local library makerspace (prior to COVID-19 shutdown) has a Prusa i3 MK2 (no MMU) that had a similar problem. The model being printed was relatively small, compared to the bed and was being printed nearly atop the temperature sensor.

The part cooling fan was cooling the bed sensor while the rest of the bed was not being used.

Moving the model away from the sensor resolved the problem. If you're printing in the center of the bed, consider to re-slice in a position offset from the center.

If you are using the PEI sheet as the library machine is/was, consider to shuttle your prints around the bed over time. The library's printer wore away the PEI from always-centered prints. It had to be replaced after about a year. The replacement is going on two years with little complications.