Canon don't guarantee that any third party products will work, or continue to work, with their cameras, so if you get a new camera body there's no guarantee that your old Sigma, Tamron, etc lenses will work with it, despite them working on your previous camera. After all, you can't expect Canon, Nikon, etc. to hold back the development of their cameras just so lenses made by other manufacturers will continue to work with them, so that's the chance you take when buying a third party lens, flash, etc. It may not work fully, or at all, when you upgrade your camera body.
This said, it may be another fault with the OP's lens that Sigma may be able to repair, if it turns out there is something wrong with it once it's been tested thoroughly in good light, remembering depth of field and trying it with and without the optical stabilisation turned on.