The error code is not always reset on success.
First and foremost, to find out whether a function call succeeded, you follow the semantics noted in the documentation. In this case, whether LoadLibrary returns a valid handle.
You've indicated that it does, so the result of GetLastError is not relevant. It could be the result from some previous call that failed.
From the GetLastError documentation:
The return value is the calling thread's last-error code.
The Return Value section of the documentation for each function that sets the last-error code notes the conditions under which the function sets the last-error code. Most functions that set the thread's last-error code set it when they fail. However, some functions also set the last-error code when they succeed. If the function is not documented to set the last-error code, the value returned by this function is simply the most recent last-error code to have been set; some functions set the last-error code to 0 on success and others do not.
I can't see anything in the LoadLibrary documentation suggesting that the error code is set to 0 on success, meaning it's meaningless in your case.