So it sounds like an issue with […] handling per-spec the presence of data which you don’t use.
The trouble is, in this specific use case, the data may either be there or it may not be, depending on authorization. I’m checking specifically if the key is present to determine whether the user has access to that data (and display a toggle for that data), or if the user mustn’t see it and thus doesn’t need the toggle.
The wrong assumption was that if the key is there, then the value is not null. That assumption was invalidated by an update on the server side. Of course I could’ve checked if the value of the key is nullish. But it’s still annoying to have a breaking frontend because the backend changed how it serves its data.