From Tamron:
Notice about compatibility between Canon EOS 5D Mark IV and Tamron lens products
Dear users and potential purchasers of Tamron interchangeable lenses.
Thank you for using Tamron products.
Dear users and potential purchasers of Tamron interchangeable lenses for Canon.
Thank you for using Tamron products.
We wish to notify you that the AF functions of the following interchangeable lenses for Canon, which have been produced before May 11th, 2015 and have not updated firmware when there were the firmware update service announcements below, do not function in Live View mode when used on the EOS 5D Mark IV which went on sale on September 8th, 2016 in Japan.
*Please see the below links for the previous firmware update service announcements regarding the EOS Rebel T6s/EOS 760D (EOS 8000D) / the EOS Rebel T6i/ EOS 750D (EOS Kiss X8i), the EOS 5Ds / the EOS 5DsR, the EOS 80D, EOS-1D X Mark II, and the firmware update service for SP 150-600mm F/5-6.3 Di VC USD (Model A011) for panning shot.
We sincerely apologize to all users and potential purchasers for any inconvenience the issue may cause.
Affected models
Interchangeable lenses for Canon, which has been produced before May 11th, 2015
Serial numbers which have produced after May 11th, 2015
Firmware update will not be required for those lenses (serial numbers).
For those who require firmware update service, please contact your local Tamron distributor for more details.