Most often, and most significantly, unintentionally, on all the issues they aren't considering when voting.

Because, actually, the policy makers they elect make policy, and and that's true whether or not the one election a single issue voter tips is enough to shift the national balance on the issue of concern.

Or whether the candidate elected on your one issue even bothers voting the way you expect on it, rather than abandoning it.

Single issue voters often don't stick with it long enough for policy makers to worry about betraying them after an election where they issue moves a small but (for reasons particular to the context of that election) decisive segment of the electorate, or if they are sticky then representatives have a strong incentive to keep them onboard with an image that victory is just over the horizon, to hold on to their vote without resolving their issue so as to enable all the other policies the representative is concerned about.