Is the antenna input the last input selected before you switch OFF the TV?
Have you tried selecting the HDMI input, connecting to it successfully, then switching OFF the TV, then switching ON the TV and seeing if it connects to the HDMI?
If it works it is 'remembering' its' last connected input and connecting to it on power on.
If it doesn't and it connects to the antenna (default) input and still maintains all the other user preference settings plus the correct TV channel information, settings etc., it may be just a glitch in the software which may be resolved by 'resetting' the TV. Be aware that when you reset the TV, it will have to be setup again, as it will be restored to a factory default condition. Like it was when you first got it.
Here is a link to the User Guide which shows how to reset the TV.