There’s two things you can be fairly sure of whenever Nikon or Canon release a new DSLR. The first is that you’ll need to wait for Adobe to update ACR/Lightroom to be able to read the raw files. The other is that some Sigma lenses will have compatibility issues, and need a firmware update.
And so, Sigma have released an advisory for certain Art and other lenses when being used with some of the newer Canon bodies. Those bodies include the 6D Mark II, EOS 77D, Rebel T7i (EOS 800D) and Rebel SL2 (EOS 200D). Fortunately, the cause of the issue is fairly specific, so the temporary workaround is quite simple.