Recently I upgrade from Test Complete 11 (32bit) Windows 7 Professional and IE 8 to Test Complete 12 (64bit) Windows 10 Professional and IE 11.
On my old environment IE opens in Fullscreen mode...
The environment changed... Therefore you SHOULD most DEFINITELY expect behavior to change unless you make specific efforts to adjust for the change in environment. UI Based automation using tools like TestComplete have a lot to do with the environment.
The solution:
1) Run IE using the exact same user profile that you are running TestComplete under and make sure it's maximized under that profile. Keep in mind that running TestComplete "As Administrator" is a DIFFERENT profile and IE is run under THAT profile. You will need to make sure that anything you do to configure your environment for TC execution is done under that profile
2) Add mitigation code to your project to account for potential change in environments. I'm pretty sure this won't be the first time. So, write a bit of code or keyword test that, when you launch IE, maximize it every time. Easy enough code to do. We have a keyword test that, at the beginning of every test case, we use to run our browser. Basically, even if we have everything configured, we have this in place to make sure that the browser is ALWAYS maximized. Simply... looks like this (see below). So... if our environment changes, we get a new OS, we get a new version of TC, we have a user profile change, etc... the browser is always maximized