From Sigma:
Thank you for purchasing and using our products.
We have found that some SIGMA interchangeable lenses for CANON are not fully compatible with CANON EOS-1DX Mark II, which was released on April 28th. When certain lenses are attached to this camera, exposure of the image may not be accurate.
We are sorry for the inconvenience, and we will make a further announcement on our website when specific details are finalized. In addition, please also refer to the below usage notice related to this announcement.
[Phenomenon]
When the lenses listed below are used and either “Evaluative Metering” or “Center-weighted Average Metering” is selected in Metering Mode of the camera, the image could show some underexposure.
[Products concerned]
We are currently working on the firmware update for respective lenses, and the release dates will be announced later.
Usage Notice for customers who are using EOS mount SIGMA lenses on a CANON EOS-1DX Mark II
We appreciate your continued support for our company and products.
Note from TDP: Any firmware fix will either require the Sigma USB Dock or a trip to Sigma's Service Center to implement. If you don't already have the dock, I highly suggest picking one up. [Sean]