If I understand, the "issue" of not tracking in Servo mode only happened at 600mm and not at all at 150mm? I'm wondering if the largest aperture at each FL has anything to do with it. I mean the problem of the lens not even trying to focus.
Can you check at other focal lengths, especially those where the aperture is still f/5.6 (I think up to approx 400mm)?
At 600mm when you pressed the button again and it did try to focus but failed, it really sounds like it just didn't have enough light/contrast on the targets?
Some people tested it and when moving the target gradually from closest to farthest it tracks fine even at 600mm. Moving between close to far targets in Servo at 600mm is very rare. I'm wondering what happens if, for example, you are tracking a bird and for a moment you are losing it and your focus point is on the sky, then back to the bird? Or if you are tracking a football player and lose them for a moment, or a much closer person pass in the frame, etc.?
Maybe someone can try these situations or equivalents just for a test? Not just at 600mm, but at several FLs.
It's difficult to compare both issues because most people compare with Canon lenses with a f/5.6 aperture at the longest FL. But if I'm not mistake the Sigma f/6.3 lenses don't have the same issue (the first) at their longest FL, whether they manage to focus or not?
Ok, I tried again but my neighbours must have gone to bed because the reflection on thd xar is gone. Instead, my distant object was a briggt street light a few hundred meters away. Grabbing focus at 600 was easier on the distsnt object than the near object. AI servo first worked seamlessly from near to far at 250mm. at 200mm it worked in both directions. Again, it's dark outside so the conditions are far from ideal.
I've just tried on a closer distant object...a street light about 20m away. I could get it to go from near to far up to 375mm (not an instant grab but at least a grab) but above that I had to bump focus...then it worked fine as expected.