According to this bug report, bumping locked questions to the homepage is by design. I disagree with this, but my question is about a different, but related, issue.
A couple of times (and today is one of these), I noticed that a locked question that is bumped to the homepage is not visible on the homepage of the website:
But it's still visible in mobile apps, i.e. on iOS:
And on Android:
Since the both apps are affected, I suppose that the problem is in the Stack Exchange API, rather than in the apps themselves.