A Mysterious Disease In New York City (1921)
It may have been an autoimmune syndrome. The Spanish Flu pandemic had officially ended, or at least the worst of it, that year. With the last cases around the winter of 1920-21.
1921-03-02 19:00:00 - The1920s Channel
So it's possible they were infected with the flu virus neurologically and symptoms showed months later.So it's possible, similar to what we're seeing with "long COVID" in the present, the virus also affected some individuals neurologically but the "signs" of this showed later.
It has been theorized that even with more apparent diseases of the nervous system, such as polio, the infection actually occurs before paralysis appears visibly.