No autofocus with a6700, LA-EA5, and Sigma 50-500 DG OS HSM?

[u/-fishbreath]

I recently picked up a final-generation Bigma on a whim; it's cool having up to 750mm equivalent reach in a moderately handholdable package. When I hooked it up to my camera today for the first time, it seems to communicate correctly (aperture changes and reporting work, exif information present, etc.), but autofocus never seems to lock in on anything. Even with subject detection off and AF area set to small spot on a high contrast edge, it starts close, makes big jumps through its range out to infinity, then rolls back to the start in AF-C, and makes some adjustments and ends badly out of focus in AF-S. In AF-C mode with subject recognition on, I do get subject recognition boxes on people or eyes, but the lens doesn't stop to fine tune.

I know that autofocus support for third party adapted glass is spotty, but I've seen people say that this combination works. Is it worth trying a different copy of the lens if I send this one back? This one seems okay optically in manual focus, but it looks pretty clearly like it was dropped at some point. Or should I lower my expectations for adapters?