Not sure any of this helps, but...

a) Stepper motors are designed to run hot. I've never seen the temperature on spec sheets, but multiple sites mention stepper motors capable of operating at temperatures of 85 to 100 degrees C. That's hot. They are intended for heat to pulled out of the mounting surface of the motor, something 3D printers rarely do a good job of.

b) Based on the data sheet, those 42HB34F08AB 06 are lower current/higher coil resistance than what at least the reprap wiki recommends for 3d printer use. This could explain why these motors run hotter than others for a given current setting on the stepper driver.

c) As Bob mentioned, you'd typically have issues with the stepper driver before you'd have problems with the motor itself. The stepper driver chips are designed to simply stop driving the motor if the chip gets too hot. Motors don't have any kind of protection built into them.

d) Be aware that once enabled, stepper motors ALWAYS have current flowing through them, and thus are ALWAYS consuming power and generating heat. The only time this isn't true is when the printer firmware has the stepper motors disabled. Once the stepper drivers are enabled, coils are energized to act as a brake and provide the motor with holding torque. This could explain why motors are cool when the printer idles with the hot end hot but then heat up in use.