Hypothetically, if the cover was 8" Dia, and a perfect vacuum was present, up to approx 360lbs of uplift could be present, for a metal cover that may weigh 25? pounds.

I question that any "kick up" forces were present by contact with a tire. Now if an item run over had a compressive/rebound response to tire contact, I might consider it, but a nearly solid rigid metal cover meant for decades of heavy traffic is not flexible nor compressible in this case, its geometric design does not lend itself to becoming dislodged by lateral forces. If the tire runs directly over the cover, a cars floor ground effects are much lesser at play at the edges away from, the tunnel. How much drop in barometric pressure exists directly behind a rotating tire at speed is unknown, as is the tire's rubber "stickiness" in the rather cold conditions.

My takeaway so far is the cover was simply not secured, and likely very obvious to first responder track crew in the minutes following the incident, and why they quickly checked the rest of the circuit, and a mistake a multi-billion dollar operation is not quick to admit as it's a huge PR fiasco.

I suspect in the future a program will be implemented that street removeable covers on race circuits will be paint marked after verification they have been properly secured, like the paint marks on critical fasteners on cars/semi's etc.