I think it partly depends on autofocus mode, EyeAF on/off, and Flexible Spot or Lock-on AF: Expand Flexible Spot (or maybe another newer mode I don't have). In any case you should be able to move it around prior to lock acquisition. Once you have acquired lock, I would think you would normally want to keep it on target, else why get a lock? Or are you trying to change what is being targeted?
So many variables. I have an older A7ii but I think the above still applies. Cameras are complicated these days.
I've been faffing about with various focus modes on my A7RIII this weekend. I can't seem to get anything to work all that well on a moving toddler! I think some settings are dependent on AFsingle or AFcontinuous. If you have any tips they would be very welcome. I imagine a moving toddler is not dissimilar to bifs!
As it happens I'm just reading Gary Friedman's book on the A9.
It's a breath of fresh air but focus modes and areas are still complex.
The lens and aperture you're using may impose limitations.
For the A9 he says face and eye AF use contrast detect mode which will be relatively slow. Turn them off, use an FE lens and AF-C, and try Tracking Zone.
Other members know a lot more about this than I do.
I only have the older A7ii and only one AF lens, and I am pretty sure you have more options on newer bodies, so take this with a large grain of salt.
I've watched some stuff by Gary Fong on auto focusing. As I recall, for tracking moving stuff, he uses AF-C and Lock-on AF: Expand Flexible Spot with his center button custom set to Center Lock On AF (might be a little off on the name). Then in the EVF, he puts the target on the subject, clicks the center button, and it tracks the subject thereafter. I am too lazy to go find it but it was on his Youtube channel from I think 2017.
I've seen demos using EyeAF on version iii bodies, but do not know what settings they used. It tracked very well. The green box would move from eye to face when it could not find the eye, then on whatever when the model went off frame, then back to face and back to eye when the model came back. It looked pretty impressive and would be well worth getting it set up. I havve used face detect which works similarly, pretty cool.
I like Mark Galer's videos. Here is one he has on sports/action setup on an A9. He recommends AF-C, drive mode Continuous High, and focus area Lock-on AF: Expand Flexible Spot.
I think there's a significant difference in AF performance between firmware 4.1 and 5.0, and Galer's links often lead you to the former. Lock on has been superceded by tracking. And despite his verbosity he can be incomplete. Eg. What does flexible mean?
Anyway, my query is simple - should a flexible spot lock be free to roam the frame?
I originally set mine up as per Mark Galer but I find that it still doesn't lock on. In fact it seems to veer all over the place, sometimes hitting the spot and then the box elongates and goes blurry and misses completely. I find it very frustrating! I have an FE55, FE85 and the 35mm 1.4 ZA. My Zeiss Batis 40mm seems the quickest of these three with the 35mm coming in last funnily enough.
One of Galer's vids, along with the Northrups and Friedman, recommend as the default leaving the area to the camera - ie. start with Wide. Galer reckons the focus algorithm will preference what's nearest and closer to the centre. If that's not your subject, then go narrower.
That said, I often try to use Spot: Small to get a bird in amongst foliage, and it's pretty hopeless on AF-S but a bit better with AF-C, and that may be the difference between CDAF and PDAF. The A9 doesn't have cross-point sensors and that doesn't help here.
This is with the FE 100-400mm and 1.4 TC.
Testing the new lens this morning, set on Tracking: Zone, I found that a lock gained within that area could indeed roam outside it. Still not sure about non-tracking settings.
I also found again that Tracking: Zone fails to lock on a distant flying bird against a busy background. That's of course a big ask, and if you really want that bird you have to stop, change AF area and get the lock the old-fashioned way by laying a small cluster of sensors on the bird.
This kind of photo isn't going to make a 'picture' but some shooters want it for the record.