Canon EF 70-200mm f/4L IS ver I USM on an R6 with adapter

My wife and I just picked up two R6s. When she is using her EF 70-200mm f/4L IS ver I USM with the EF to EOS RF adapter, the lens does a lot of AF noise and seems to be hunting hunting and does not work very well with the Animal Eye AF. Her R6 seems to work well with her EF 100-400 II and the kit RF 24-105 non L. I am assuming it may have something to do with the lens being relatively old technology. Any thought on this?