Focusing on a subject's eye is a much easier target for the AF system as it usually presents a high contrast area - the centre of the eye is normally much darker than the surrounding skin and in any case has a dark centre surrounded by white. Your banana was a low contrast target and with lighting levels that require ISO 1000 and f/1.4, there is very little contrast variation for the AF system to detect.
I've shot portraits with my 50mm at f/1.4 - f/2.0 and got the eyes bang in focus very easily. Depth of field is so shallow at these settings, that the ears and nose are out of focus, but the 50mm f/1.4 has always got the eyes tack sharp.
For proper AF fine tuning, you need to optimise the focusing conditions for the lens - that means using good light and a high contrast target.
Your banana was shot in low light and was a low contrast target.