Here's the lowdown, including an illustration:
1FZFE Thermostat gasket replacement - https://forum.ih8mud.com/threads/1fzfe-thermostat-gasket-replacement.1222522/
Most good auto parts stores have a test kit available for checking your coolant to see if there are signs of a HG failure.
Recheck that thermostat to be certain is installed correctly. If they are just following the way they found it, it could be wrong. The orientation of the t-stat and the placement of the O-ring can cause issues.
It may also be a good time to get a test kit to check about the possibility of a HG...