Sigma Japan issued an official statement:
For customers using SONY α9 (ILCE-9) camera
Thank you for purchasing and using our products.
We have found that the combination of SIGMA MOUNT CONVERTER MC-11 (SA-E and EF-E) and some of SIGMA’s interchangeable zoom lenses may not focus precisely, if it is attached to SONY a9 (ILCE-9), released by SONY in May in 2017.
When using AF-S mode and shooting in telephoto range, it could occasionally display the shot is in focus, although it may not be spotted properly on the subject.
We are afraid of this inconvenience caused to the customers who use these products.
We are now investigating the phenomenon and will make a further announcement on the website shortly.
We appreciate your continued support for our company.For further information, please contact your nearest authorized SIGMA subsidiary / distributor.
http://www.sigma-global.com/en/about/world-network/