I've had the R5 with RF100-500 for a year with very impressive results. That being said, the R5 with the RF100-500 has a terrible time with BIF when there are trees in the background. When shooting photos In the blue sky, no problem, the animal eye detect + tracking nails it. White bird, dark bird, doesn't matter. When shooting into the blue sky, it gets the bird. But when there are trees across the horizon in the background, call it camouflage, it can't find the bird. Even when the bird is large and near (e.g. Egret or Osprey at 50 yds), the AF will not acquire. It searches the camo background but can't find the object.
Is anyone else having this problem?
The gear will only do what you ask it to do. If you ask the wrong questions, then you will get the wrong answers. You did not post a sample photo of your issue. I recommend posting a sample photo to help others to help you.