We're talking about complete focus failure (Weakness 1).
I found that AF-C appeared to work a little better than AF-S. Reading David Busch's guide to the R III, he says AF-C operates predictively. It may be that lacking a quick lock it does as you say and settles for the distant lock. I don't think it's switched to CDAF at this point since that's obvious by its slowness and hunting.
There are a few generalisations as to the subject matter I can make. They're tentative ones but we can test your hypothesis against them.
It may be more likely with a monochrome subject.
Sometimes I think it's less likely if AF is set to tracking.
It is less likely if the subject moves a bit.
It may be more likely if there isn't a good contrasty edge in the focus area but good light and a contrasty edge are not a guarantee of success.
It almost always happens with a kind of fence post we have here called a star post which is brown to black steel about 40mm wide say 5 to 10 m away (in my area a bird is often to be found perching on the wire near such a post but I can't use the post as a focus springboard).
So I think your hypothesis is plausible.
I have a couple of buts though ...
Sometimes the Small Spot is entirely on the bird and there's no more distant point to focus on.
Some of the completely failed focus shots' EXIF recorded no AF point but what's in the distance looks sharp or sharpish. So there may be an infinity default (when in doubt, rack it out).
It may be worth trying the middle focus range limiter and see what that does (on the lenses that have it). As a bird shooter swapping to that when presented with a subject will lose me shots, and I can't accurately estimate 5m or 7m or whatever distance away, but it may add to insight. (As an aside I'd like to see that setting and OSS mode software selectable and so assignable to a custom button.)
Some folk suggest manual focus but I don't find it easy with a 4.3 kg rig. It's slower than successful AF and loses shots too.
Re getting a kind of prefocus using AF, that's what I've been doing - finding something nearby as a springboard.
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I'm going to add a Weakness 4 which may be related to 3.
Sep 21, 2020 at 05:25 PM
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