Coolant low?
If it gets back to normal after adjusting the level, I would suspect that low coolant was the cause. What caused the coolant level to be low? You say that you've replaced the "Water Neck" with an aluminum one, so I'm assuming you mean the "water Inlet" on the driver's side above the transmission? Ensure it isn't leaking, and then proceed to the Thermostat housing.
My Mother-in-Law has a 2016 Buick Encore, which has the same 1.4L turbo as the Trax.
I've had to replace the Intake Manifold, the tube from the intake to the turbo, and the Valve Cover. (PCV issues that are guaranteed in these things.) Then I've had to replace the Thermostat housing and water inlet. These were the reasons for the coolant levels dropping in the reservoir. These all developed coolant leaks months apart from one another.
As of yesterday, I've had to deal with a P0234 (Overboost), which I traced back to the "Wastegate Solenoid." Usually, it's a P0299 (Underboost) in these things.
I believe I'll have to deal with the infamous oil cooler housing leaks sooner rather than later. I was hoping to wait until the turbo bit the green weenie before tearing into that. I suppose the 5K/6-month (whichever comes first) OCI's with Mobil1 EP 5w30 have kept the turbo from eating itself.
These things are garbage!
Sorry, I hate the GM 1.4L with a passion, and I will trash it every chance I get. They have no redeeming qualities!
OMG, my blood pressure is getting up. I want to kick a GM engineer in the gonads right now!