I believe I'm having a related issue using ES 5.4.0. I have a date field which comes back with sort value 9223372036854776000 when missing. Using that value in search_after gives the error: No matching token for number_type [BIG_INTEGER]
I haven't found a workaround yet. It seems like search_after currently can't be used when sorting by an optional date field.
Hopefully someone can help determine if there is a fix or documentation update in order.