Tamron has warned its users about the problem with autofocus of the E-Mount lenses: 20mm F/2.8 Di III OSD M1:2 (Model F050), 24mm F/2.8 Di III OSD M1:2 (Model F051) and 35mm F/2.8 Di III OSD M1:2 (Model F053). When using these lenses with a Sony a7 IV camera, involuntary switching from auto focus mode (AF) to manual focus mode (MF) occurs. The manufacturer acknowledges the error and will try to correct it. Perhaps Tamron will update the firmware and this will solve the problem.
At the moment, the lenses have a firmware version Ver.2 20.07.06 (it was released in the summer of 2020).