A few years ago I found myself in an odd situation. One of our PMs was not doing their job: there was not enough planned work to keep engineers engaged. This was leading to second order effects: engineers were demotivated. Beyond individual needs, the product was stagnating. A peer of mine and myself proposed that we would act as PMs for a short period of time, so work could continue. This work, was on top of my existing duties as an EM. We did so and stuff started to get done. If my peer or I said “that’s not my job”, we would risk engineers resigning and moving somewhere else. It was definitely not in my job spec to do PM work, but there I was writing product specs and tickets.
Thiago is correct that the higher you go in seniority, the more is expected of you. Shit happens. Unplanned situations pop all the time. Expecting a job spec to meticulously describe the exact functions is unreasonable. If one doesn’t cope well with unpredictability, there are ways to minimise it (e.g. work in large, well established companies).
As I later tweeted, a developer saying “it’s not my job” is a red flag. Identifying a gap is the first step to filling it and this can be done by anyone. The least amount of effort, is to simply make your manager aware of a problem. It doesn’t imply that you are the one fixing it, or having to work the extra hours. At a senior level (and to reach such levels), traits like initiative, leadership, resilience and team work are greatly appreciate and in short supply. If your employer doesn’t value such things, there are plenty others that will.