From Canon USA:
Future Firmware: EOS-1D X Mark III
Thank you for using Canon products.
We have confirmed that the following phenomenon may occur during use of the EOS-1D X Mark III digital SLR camera. A new firmware release is planned for early April; once the new firmware is ready, we will make an announcement on our website.
We would like to offer our sincere apologies to users of these products who may have been inconvenienced by this issue
When the camera is used with the electronic level set to “Show” in the viewfinder display, in rare cases, the camera may not perform normally when operating the AF-ON button or shutter button.
EOS-1D X Mark III (Firmware version 1.0.0)
Use the “Hide” setting for the “Electronic level” in the “Viewfinder display”.
Menu / Set-up 2 tab / Viewfinder display / Electronic level* [Hide]
*Default setting: [Hide]
In early-April 2020, we plan to release a firmware update to address the phenomenon listed above. Once the new firmware is ready, we will make an announcement on the Canon U.S.A website.
In the interim, please follow the steps provided in the workaround.
If you have not already done so, please register the EOS-1D X Mark III. By registering, we will be able to notify you via email about future announcements.
Check out our full Canon EOS-1D X Mark III review
and order yours now