The Russians were not attacking U.S. forces; U.S forces were attacking ISIL. The U.S. was in contact with the Russian liaison at Deir-Ez-Zor who said there were no regular Russian military present. The liaison did not mention contractors, and the contractors were not told a attack was imminent.
No, the "Russian" forces, actually a mix a mercenaries working for a Russian contractor, were attacking US troops, and they knew it. And the US troops knew the attackers were Russian from SIGINT. We (US military) repeatedly contacted the Russians via an established "hotline" to try to get them to withdraw under threat of overwhelming force. The Russians repeatedly denied any involvement in the assault, so we let loose the dogs of war. It appears either the Russian military didn't know what Wagner group was doing, or was willing to let them continue with the assault, thinking they would succeed.