Avoiding death is a primary goal of medicine. Avoiding side effects of treatment is a secondary goal. So it makes sense that in a far mode doctors emphasize avoiding death, but in nearer mode avoiding side effects matters more
An alternative hypothesis*: the vividness of death doesn’t increase as much as that of suffering in near mode because it is relatively hard to imagine in detail. It basically consists of you not being there, which is pretty nonspecific with regards to other details, and the details you can picture are not ones you have ever experienced or anticipate experiencing. Suffering on the other hand is a very familiar experience with details generally considered too vividly memorable. Particularly when the suffering is of a specific kind, such as the chronic diarrhea used in the study. I expect most people can picture chronic diarrhea in much more horrible detail than they can ceasing to exist.
How to tell between these hypotheses? I expect advisors make the same kinds of trade offs for themselves and others in situations where avoiding death and avoiding injury both seems like secondary goals, for instance in sports coaching and military tactics.
*Based on construal level theory – the idea that we think differently about things that are near and far from us. See more about how we think differently here. See also the potential relevance of this to cryonics, procrastination and euphemisms.