This sounds like you have an air pocket. If the thermostat was remaining closed then you would not have heat in that hose as nothing would be coming through and that include the heat.
So I would suggest topping off the coolant and jacking the front corner of the vehicle closest to the fill cap so that it is higher then the rest of the system. This will help work the air to the highest point.
Here is a video on a similar video that shows how to service the system:
Also, here are some other guides that help with these issues:
Lastly we need to pressure test the system to make sure there are no leaks and that it can hold pressure. Here is a guide for that:
I suspect if anything that you got a faulty thermostat you may end up replacing it again. I attached the process if you need it. Thanks
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Tuesday, December 15th, 2020 AT 2:14 PM