Hi, my name is Rich & I'll try my best to help you today.
This is a hard question to answer in this type of forum since I cannot hear or see the vehicle.
There are a lot of things that could have happened when they changed the thermostat.
There have been times when my big hands and/or elbows (when digging down in an engine compartment) have knocked vacuum hoses off or accidentally unplugged a sensor.
It's not that we do it on purpose, it's just that the manufacturer crams everything into a tiny space never taking into account someone is going to have to work in there.
As for making this new mechanic your permanent guy, there's a couple of things to look at:
1. Is he charging you to take a look at it again?
2. Was he courteous when you called in about the complaint?
3. Did he immediately deny that it has anything to do with his work? (see accident comment above) I never do - I might have knocked something loose.
4. What is the resolution? By this I mean: Wait & see what he says tomorrow and see if it makes sense. If it does not, then by all means, contact me back with what he says and we can go from there.
I hope everything goes well for you and that there is an easy solution for both of you to build the rapport for a lasting relationship.