The current issue, which effects Sigma’s lenses when paired with Canon’s 5D Mark IV, stems from the lenses being incompatible with Canon’s manual focus override feature (which allows you to manually refocus when AF is on). There is no word on if or when a fix is expected, but for now if you own Sigma glass and want to manually focus on a new 5D Mark IV you will need to specifically set the camera/lens to manual focus or you are likely to run into issues.
The other thing noted in the release has to do with a long-running issue experienced by some users in which Canon’s lens corrections cause some odd issues with Sigma lenses. There is still no fix for that other than to turn the lens correction off in your camera, Sigma noted that this affects the entire EOS system (to varying degrees, some users have the issue happen, and some don’t), and not just the 5D Mark IV.
You can read the full memo below, or over on Sigma’s website, here.
Dear Canon 5D Mark IV Users
Thank you for purchasing and using our products.
Please find the following notes on the usage of SIGMA interchangeable lenses for EOS, when attached to Canon EOS 5D Mark IV that is released on September 8th.
・ MO (Manual Override) function is unavailable with this camera (Full-time Manual function can be offered).
・ When using a SIGMA interchangeable lens for EOS, setting the corrections to [Disable] is recommended, as “Lens Correction” functions of the camera, such as Peripheral illumination correction, Chromatic aberration correction, Diffraction correction and Distortion correction are not supported.*
* If those functions are activated, the performance of lenses may not be accurate.
* This is not only for EOS 5D Mark IV, but also for other camera bodies with the same function.
We appreciate your continued support for our company and products.