We always take it into account. There may be situations like that. However, in my experience in palliative care, it is a request that has been thought about, often for a long time. So it is a process that takes shape in the person's mind.
Have I encountered situations in the past when the person was unable to receive palliative care? Yes, of course, that has happened. However, as you say, in a majority of cases, in about 80%, the patients receive both types of care, and that is what we want: we truly want everyone to have access to both options.
Some people may have made the decision after thinking about it for several years, while others arrive at this conclusion because they are suffering too badly in spite of palliative care. I would remind you that suffering can be psychological, also. Waiting for death is psychologically very difficult for some people, so they decide to turn to palliative care. While the usual palliative care relieves pain and symptoms, existential suffering is more difficult to relieve, even when the palliative care is optimal.