I think I've found the issue now. I think my forehead was slightly catching the multi point controller when in portrait mode.
This means that when I half press the shutter it moved the focal point.
I've turned the multi point controlled off focal point now, and it seems to be better. I'll try it properly tomorrow but hopefully my issue is resolved!
This happened to me with my 650D when I was shooting in portrait with the battery grip. I thought there was an issue with the grip, but my forehead was hitting the other focus button which cancelled out the focus button on the grip. Big facepalm when I figured that one out.
I have noticed my 6D and 85 1.8 are front focusing, so I'm going to try MFA tonight.











