Bird takeoffs are tough. They explode so fast it is hard for the AF to kick in. If those shots were sequential in five frames a whole 1/3 of a second elapsed +/-. That's pretty demanding. Also here is way to much going on otherwise in that setting that would undermine you. I haven't gotten a chance to test the R5 with a take off or landing like this one, but those type of shots are one of my favorites and I have been looking for a chance to test it.

Typically though with the DSLR's the bird might fall out of focus for an instant but once the camera caught up it would track. You saying it progressively got worse is concerning.

One thing I have noticed with the R5 when using just a single point for focus it has a harder time picking up a subject in the bush than some of the DSLR bodies. It seems like the single point is fat. Maybe there is a setting I am overlooking, I will just keep playing with it until I figure out if it is my settings or the camera.