Our FSA (Flex plan) allows you to carry over into April of the next year. Excellent, excellent benefit.
FSA plans typically allow 15 1/2 months to expend funds before they are lost. What congress doesn’t allow is for you to continue to build up those funds in perpetuity. There doesn’t seem to be any good reason for them not to allow this, except that they don’t want people to have a large sum built up to use for a rainy day. People taking care of themselves is anathema to the statists.