Why Activity.runOnUiThread(Runnable) stops executing the Runnable?

Question

I am programming an EBookReaderApp for Android. It has an automatically scrolling function. The scrolling is implemented in a separate thread to not block the UI all the time. The actual scroll has to run on the UI thread. Therefore I call Activity.runOnUiThread(Runnable) So far so good. After some time (sometime as low as 2 minutes, sometimes after 1 hour) the Runnable stops being executed. Has somebody an idea why runOnUiThread stops executing things completely?

private class ScrollThread implements Runnable
{
    @Override
    public void run()
    {
        android.os.Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
        boolean initialised = false;
        long lastCall = System.currentTimeMillis()-25;
        Runnable r = new Runnable()
        {
            public void run()
            {
                log("b");
                content.scrollTo(0, (int)scrollValue);
            }
        };
        while (!kill)
        {
            long diffTime = System.currentTimeMillis()-lastCall;
            lastCall = System.currentTimeMillis();
            float fps = 1000f/diffTime;
            if (!initialised)
            {
                if (initialisedChapters == chapters.size())
                {
                    if (content.getHeight() > 0)
                    {
                        initialised = true;
                    }
                }
            }
            if (initialised)
            {
                if (!scrollBreak)
                {
                    scrollValue += content.getHeight()/fps/secondsPerPage;
                }
                recalculateChapter();
                log("a");
                runOnUiThread(r);
            }
            try
            {
                diffTime = System.currentTimeMillis()-lastCall;
                long sleepTime = 1000/40-diffTime;
                if (sleepTime > 0)
                {
                    Thread.sleep(sleepTime);
                }
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
        }
    }
}

logfile:

2022-01-20T14:35:32.109+0100 a
2022-01-20T14:35:32.115+0100 b
2022-01-20T14:35:32.145+0100 a
2022-01-20T14:35:32.151+0100 b
2022-01-20T14:35:32.179+0100 a
2022-01-20T14:35:32.185+0100 b
2022-01-20T14:35:32.210+0100 a
2022-01-20T14:35:32.218+0100 b
2022-01-20T14:35:32.246+0100 a
2022-01-20T14:35:32.254+0100 b
2022-01-20T14:35:32.276+0100 a
2022-01-20T14:35:32.284+0100 b
2022-01-20T14:35:32.309+0100 a
2022-01-20T14:35:32.330+0100 b
2022-01-20T14:35:32.344+0100 a
2022-01-20T14:35:32.348+0100 b

[...]

2022-01-20T14:55:39.440+0100 a
2022-01-20T14:55:39.447+0100 b
2022-01-20T14:55:39.475+0100 a
2022-01-20T14:55:39.482+0100 b
2022-01-20T14:55:39.510+0100 a
2022-01-20T14:55:39.515+0100 b
2022-01-20T14:55:39.545+0100 a
2022-01-20T14:55:39.580+0100 a
2022-01-20T14:55:39.616+0100 a
2022-01-20T14:55:39.651+0100 a
2022-01-20T14:55:39.678+0100 a
2022-01-20T14:55:39.713+0100 a
2022-01-20T14:55:39.746+0100 a
2022-01-20T14:55:39.781+0100 a

It keeps printing "a", "b" is never printed again... I am absolutly clueless and did not find solutions.