Update Matching records in found set
What was your found set in the target table at the time that you imported?
It may be that you need to do a Show All records on the target table occurrence before importing to make sure that all records in the target table are also in the found set.
Phil, Phil, Phil - why didn't I post this question a YEAR ago! That's the problem. In most cases, I always had some found set from searches prior to uploading. I guess I just never paid attention, or assumed that it would still search everything, not just the current found records.
Can't say 100% if that fixed it, but for the first time in forever, the import just worked perfectly on first try after doing "show all" records.