Snape was ultimately able to explain it away though:

‘I think you next wanted to know,’ he pressed on, a little more loudly, for Bellatrix showed every sign of interrupting, ‘why I stood between the Dark Lord and the Philosopher’s Stone. That is easily answered. He did not know whether he could trust me. He thought, like you, that I had turned from faithful Death Eater to Dumbledore’s stooge. He was in a pitiable condition, very weak, sharing the body of a mediocre wizard. He did not dare reveal himself to a former ally if that ally might turn him over to Dumbledore or the Ministry. I deeply regret that he did not trust me. He would have returned to power three years sooner. As it was, I saw only greedy and unworthy Quirrell attempting to steal the Stone and, I admit, I did all I could to thwart him.’

- HBP, 2