I haven't heard of this issue before, but a quick research showed me that other people have (similar) AF problems with longer lenses on an R6 II. See here https://community.usa.canon.com/t5/EOS-DSLR-Mirrorless-Camer...
Sorry for not providing an explanation, perhaps I can find some time to look deeper into this - but I can't promise.
I haven't heard of this issue before, but a quick research showed me that other people have (similar) AF problems with longer lenses on an R6 II. See here https://community.usa.canon.com/t5/EOS-DSLR-Mirrorless-Camer...
Sorry for not providing an explanation, perhaps I can find some time to look deeper into this - but I can't promise.