In my experience, high-current die are thinned from 300 micron down to 100 micron. What does that tell us, in the absence of die-substrate doping info? not much.
But the thermal behavior is still available, if we perform a tiny bit of physics.
The thermal time constant of a cubic meter of silicon, with heat entering one face and exiting the opposite face and the other 4 sides being thermally insulated, is 11,400 seconds.
What about 100 micron cube?
The thermal time constant of 0.1 meter cube is 100X faster, at 11,400/100 = 114 seconds.
Sketch a cube, slice it into 1/10th cubes in all 3 dimensions, and you'll observe that 100X speedup; volume is down 1,000X, but heat path resistance is up 10X, hence the 100X smaller thermal time constant.
The thermal time constant of 0.01 meter cube is 100X faster, at 1.14 seconds.
The thermal time constant of 0.001 meter cube is 100X faster, at 0.0114 seconds.
And the thermal time constantof 0.0001 meter (100 micron) cube is still another 100X
faster, at 0.000114 seconds (or 114 microSeconds).
Thus in 100 micron-thick PowerMOSFET structures, these long pulses are no longer thermally-absorbed within the die, and the die (and the metal mounting plate) begins to heat up and allow all the various temperature effects, mentioned by Neil_UK and by Peter Smith, to cause problems.