Rolled back to 1.4.0 and my R5 is reborn.
R5 with 1.4.0 and 1.5.0 are two different cameras.
1.5.0 keep focus-jumping when there have many birds in the frame or solo birds with moderate background. It jumps focus point unreasonably to something else, a bird or a tree, whatever it is, hardly keeps the tracking as constancy as it should be.
Another issue is focus hunting. It happens more than before.
The keep rate for birding from 1.5.0 is simply noticeable lower than previous version.
I noticed immediately the difference from the first BIF shots after downgrade back to 1.4.0, the small box follows the bird all the way from the sky until disappeared in the bushes. It's impossible for 1.5.0 to do this, however I do the settings, it would have already jumped the focus at the halfway.
Canon must have trained R5/6 to recognize more. Not only get to know a vehicle, but also to focus with environment aware. Problem is too good to handle, forgot the stability of focus tracking is more important than what can be tracked.