If this was PPO I would suggest that it was the std logits for the variance approaching zero. This problem has been reported many times. I have not seen it arise as an issue with SAC though. And I do not know enough of the RLlib implementation details of SAC off the top of my head to know if it is likely the issue.
There is an easy test and fix if that is the problem. You can set this in the config. model_config_dict{"free_log_std": True}