R7 Autofocus Hunting Issue with Tamron 18-400mm Lens

[sonoma]

I've been having consistent autofocus issues trying to use my R7 with the Tamron 18-400mm lens using the control ring EF-EOS to R mount adapter.  The lens will often work fine for the first 5-10 minutes of a photo shoot, but then it will start hunting for focus and be unable to focus on anything.  Turning the power off and back on doesn't help.  The firmware on both the camera and lens have been updated to the latest versions.  I shipped the lens to Tamron for troubleshooting and repair, and they adjusted the autofocus, but that hasn't resolved the problem.  The lens works fine on my older Rebel T4i that has the same EF-S mount as the Tamron lens so no adapter is needed.  Is it possible that the mount adapter is defective?  Or does the R7 with the mount adapter just not work properly with the Tamron lens?  According to Tamron, the R7 is supported with this lens.

[sonoma]

Hi Trevor,

Thanks for your reply.  Tamron has a lens/camera compatibility page at https://www.tamron.com/global/consumer/support/checker/ that specifically says that the 18-400mm lens is compatible with the R7.  I checked that before I purchased the lens.

I am mostly using single shot mode with the lens, not high-speed shooting, so max frame rate or full buffer should not be any issue.  My problem is with inability to focus to take an initial shot, not incorrect focus on multiple shots.  The lens fails to focus on anything so I get the red focus indicators instead of green and the image in the viewfinder is completely blurry with nothing in focus at all.  So I think what I'm seeing is quite different from the issues that you're describing or what is mentioned in the video by Duade Paton.