You're assuming that when the premium is deducted, it's rolling forward for the pay period still to come. That is not always the case - in fact, it is rarely the case. Far, far, far more often when the premium is deducted it is for coverage in the same pay period that the check is for. You are not paying for the coverage in advance; you are paying for it as you go.
If that is the case, and it very likely is, then you are able to use the coverage until the premium is used. It's just that it's not being used beyond the last date of employment.