The problem we were hitting is that a remote removal caused the contact to be removed from the mutatedPrevRemote list - which means that there is no new data to calculate the new localSince value from.  Hence, in that case, it would revert to the old local since (or, actually, in the code, it would revert to a point prior to the last local since, but functionally those two things would cause the same outcome).

We need to ensure that the timestamp we use for this invocation is after the createdTimestamp of the locally-created contact which no longer appears in our exportedIds set.