This article argues that Wilson had serious issues well before his stroke:
Wilson’s judgment seemed grossly impaired by the war years. He was extraordinarily petulant and irrational by 1918, and contemporaneous observers who were in a position to know commented often on his strange and quirky ways.
In 1919, Wilson’s pre-existing medical and mental conditions arguably led to a breakdown months before his paralytic stroke, which occurred on October 2. The nature of this breakdown could be seen as early as February, in a series of words and actions that prefigured his behavior of November and December, at which point he was clearly out of his mind.
When Wilson sailed to Europe aboard the USS George Washington, he had — typically — no substantive strategy for preventing the kind of vindictive peace that he had warned against in his 1917 “Peace Without Victory” speech. One of the advisers recruited for the U.S. peace delegation, Yale historian Charles Seymour, recalled that Wilson turned to him during the voyage and asked, “What means, Mr. Seymour, can be utilized to bring pressure upon these people in the interest of justice?” It was very late indeed for Wilson to be thinking in these terms, especially after the many missed opportunities in 1917 and 1918 to build the political pre-conditions for “peace without victory.”
This talks about a major shift in his health and behavior:
...something drastic seemed to happen to him on April 28 — something that did not come to light until many years later, when historian Arthur S. Link was editing the Wilson documents from 1919. Let Link and his editorial colleagues tell the story: “It became obvious to us while going through the documents from late April to about mid-May 1919 that Wilson was undergoing some kind of a crisis in his health . . . . Whatever happened to Wilson seems to have occurred when he was signing letters in the morning of April 28” when his handwriting changed and became almost bizarre.
The editors continue: “Wilson’s handwriting continued to deteriorate even further. It grew increasingly awkward, was more and more heavily inked, and became almost grotesque.” Link summoned some medical specialists who told him that in their own opinion there was simply no doubt about it: Wilson had suffered a stroke on the morning of April 28.
And then he threw away yet another opportunity to strike a blow for “peace without victory.” When the terms of the Versailles treaty were made public there was widespread outrage regarding their severity. David Lloyd George, the British prime minister, was stricken, and he called the British delegation together on June 1. Their decision was unanimous: the terms of the treaty should be softened.
But when Wilson was approached, he declared that the severe terms were perfectly appropriate. According to one account, he proclaimed that “if the Germans won’t sign the treaty as we have written it, then we must renew the war.”