Does ignorance that returning saliva from the outer lip into the mouth invalidates the fast mean that the fast is not broken, and what should one who has done that frequently during fasting in previous years do?

Merely returning saliva from the outer lip to the mouth does not invalidate the fast unless it is swallowed intentionally. If it is swallowed intentionally, then the fast is broken.

However, if one swallows it out of ignorance that it breaks the fast, then their fast is valid according to the view that knowledge is a prerequisite, which is the correct view based on the indications from the Quran and Sunnah that knowledge, remembrance, and intention are prerequisites.

Even according to the view that ignorance is not an excuse, except for a new convert to Islam or someone who grew up in a remote desert far from scholars, some jurists differentiated between ignorance of a fast-breaking act that is commonly known among most people and ignorance of a fast-breaking act that is obscure to the majority.

So, if someone expelled saliva and then swallowed it out of ignorance, their fast is not invalidated. However, if she did not grow up in an ignorant environment far from people of jurisprudence, then she must strive and fast the number of days she predominantly believes she committed an act that broke her fast.