Around 2 years back I replaced the thermostat and housing and in the process, the top bolt that holds the housing to the engine broke off. I was still able to bolt the new housing with the remaining 2 bolts, which at the time I thought would not be an issue. Fast forward a year, the car started leaking coolant from the housing after a couple loops of hard driving. I kept it parked for around a week until my mechanic could come and see it. He checked it and when he test drove it for 20-30 mins and left it on idle for another 30 mins, it did not leak. Fast forward to now, It has began leaking again, but similarly to last time, the leak happened after hard driving. 

Whenever the coolant leak would happen, I would turn the car off and wait for the white smoke to dissipate. As soon as the car cooled down, it would no longer leak coolant. Heat blows warm like usual, so I don't think it is the head gasket. The first time it happened, I saw the temp gauge hover between H and the mid point, but the gauge was not moving closer to H. Still, I turned the car off to let it cool. The second and most recent time this coolant leak happened, I saw the white smoke coming from the hood and pulled into the parking lot to let it cool (car was still at operating temp and did not overheat).  

Is this all due to the broken bolt? or this there a bigger issue? P.S. my mechanic did a coolant pressure test today and it was indeed leaking from the thermostat housing.