I've had the same problem with the r5. Simply put, when I point the camera directly at a motionless small bird with a busy--but distant--background (e.g., leaves) using a single focus point, the r5 often focuses on the background instead of the bird. I have considerable experience photographing birds, mostly with a 500mm lens and a 1.4X teleconverter. I have tried every possible setting/configuration on the r5 and the problem still occurs when the bird is small relative to the background. This occurs sufficiently often under conditions where the 7d2 has NO problem focusing on the bird, that the r5 is not usable for small birds unless the background is featureless (e.g., sky). The r5 is great for bigger birds--especially those in flight, for which it truly excels.
Today I did a very simple test with a tree limb that diminishes in diameter but has other limbs and leaves behind it. With the 7d I can move progressively down the limb focusing over and over as the limb gets smaller and smaller, and the 7d2 never loses focus. In contrast, the limb becomes too small for the r5 and the r5 jumps to focusing on the background--even though the single focus point is still centered on the limb.
Here are the specific r5 settings I have found that work best for small birds, but still fail:
AF operation: SERVO AF
AF method: AF[o]
Subject to detect: Animals
Eye detection: Disable (I assigned this to the "AF-on" button and press when appropriate)
Tracking sensitivity: -2
Accel/decel: -2
Initial Servo AF pt for (face, box): [o]
Sad that I spent so much $ on a camera that does not work for my main application.