 Solution

Use the A-group observed-minus-expected convention for the stratified log-rank statistic. At the first failure both teeth are at risk, so A has one observed event and null expectation $1/2$, giving $+1/2$. At the later B failure, A is no longer at risk; the A contribution is zero. This patient's contribution is $+1/2$, favoring B. The opposite group convention reverses the sign but gives the same test.