Safety is always a priority! Thanks for bringing this up; it may indeed be a problem.
Normally when a thermistor wire breaks completely, the indicated temperature drops to zero. Technically, a thermistor of the type we use is 100k ohms at room temperature, and the resistance goes down significantly as the thermistor gets hotter, or up if the thermistor gets colder. So technically, an infinite resistance corresponds to an undefined temperature well below zero, but the table of values in Marlin firmware stops at zero (Prusa firmware is based on Marlin).
Normally, if the firmware detects a temperature of zero, it will declare a fault and stop the print.
You could try unplugging the thermistor from the board and see if the indicated temperature drops to zero (and/or causes the board to report a fault). If it does not, then there may be a problem in the firmware configuration because open circuit is definitely a fault. If your indicated temperature when unplugged does drop to zero, that probably means that in the earlier case your broken cable was not truly and absolutely broken but was maintaining a weak connection of about 100k ohms.
I can't run the same experiment for you because my MK3 uses a different board and therefore also slightly different firmware.