I think it was my mistake that I replaced the miner fan with another fan to lower noise level, when I woke up the next day, the fan was not spinning, but the firmware should have stopped mining once 80C was crossed right?
It won't stop mining at 80c, not even at 100c, or at least this is what the default firmware allowes you to go to, since S9 has a max temp of 135c 80c is pretty cool, and i have seen those S9 run above 100c for a very long time without a hitch, now going 200c is a different story, I believe that some of these custom firmware have an option to disable the fan-check function in cgminer use for immersion cooling) where fans are unplugged anyway, so maybe it was activated by default or you have mistakenly activated it? hard to tell, you have to check with the firmware devs to confirm that.
Now back to your T17e it does say "PCB max. 80" but that is far form reality, all these 17 series have a terrible solder that will fall-off with the slightest increase in temps, there is no exact figure that these gears will run safe, but the cooler the better, i believe your readings of "PCB temperature as 51C" is not bad at all.