Sony A6400 AF-c issue

Hello,

I'll include a long and short version to this issue, if this is something you have experienced or have any ideas on, I'd really appreciate any help you could give.

The short:

my sony A6400 freezes the autofocus when I press the shutter button, using adapted Canon lenses and AF-c.

The long:

I recently bought a Sony A6400 with a Metabones EF to E mk V adapter to use underwater and as a second body on land with a Canon 400mm f/5.6L lens. I photograph wildlife, with the majority being Common terns at the moment, these are small, fast birds and looking through the viewfinder I have been pleased with the speed and tracking capabilities of the camera. After looking at the pics on my computer however, I found that the number of images that are actually in focus is quite low, and after a bit of testing I think I've found the issue. It seems that when I press the shutter button, the autofocus stops tracking, essentially freezes where it sits, and only resumes when I take my finger off the shutter.

I have the camera set up with back-button autofocus, using AF-c, and either wide or zone autofocus, with the adapter in green mode (blue light) which I've found to be far better for my use. As I said tracking is fine when following a subject, with green focus squares lighting up around the subject but as soon as I press the shutter, the squares stick wherever they are in the frame and autofocus stops tracking. This makes burst shooting useless as in each successive frame the subject becomes progressively softer.

It's not that the autofocus isn't working altogether, as slow moving subjects are razor sharp and first frames of a series are good too, just quick subjects and long bursts.

The adapter has the latest firmware, as does the camera. This is my first Sony camera and the menu and it's many options new to me but I've gone through them and can't see any settings that explicitly says "cripple AF-c on/off" or anything of the like but perhaps a more experienced Sony user could point me to something I've overlooked? I've also tried some of my other Canon lenses with the same result, no Sony lenses as I don't have any.

Thanks