I just replaced the thermostat on my 190E about 3 months ago, and am wondering if they are so fragile to break like this?

my temps used to run at 80C or less, until i replaced the tstat with a new one and then it ran around 87C under normal driving conditions.

today, I ended up idling in traffic for around 30 mins, and it was hot day, so the temps were consistently near the 100C mark. i parked the car finally to get some dinner, and on the drive back home, I noticed that my temps were back to the 80C mark again, and sometimes even lower. Now i suspect that the tstat is stuck open again.

are these thermostats so susceptible to being stuck open like this just from exposure to high temperatures?