I know one that was rapid dementia. The doctors said she would not recover. Went from old and ok to psychotic and unable to feed herself in 9 months. Odd. Now she is starting to get better. Never heard of dementia patients rapid decline and start getting better.
There are plenty of weird conditions and viruses that are little understood, but have symptoms that are the same as irreversible conditions. Some sufferers can recover and some can’t.