This could be a plausible case for rollback yes, if the power fails between the other members getting the command and writing to disk.
In this case, as you point out, the primary could not start back up and so would, once back up, contain operations that the rest of the set could not validate causing a rollback.
It is also good to note, as a curve ball that if the primary were not to go down then it would return a successful write and the application would be none the wiser that the set has gone down and their {w: majority}
wasn't written to disk. This is, of course, an edge case.