Import skips silently if the import has an external identifier and the existing contact does not

A customer is experiencing issues when trying to update existing contacts when external identifier is part of the import set. I'm seeing 2 things

1) if the import HAS an external identifier and the existing contact does not then the record is skipped (even if the dedupe rule is otherwise a match).

2) If the import has an external identifier and it matches a deleted contact the deleted contact is undeleted.

I'm unsure if 2 is correct - maybe it is but for 1 I would expect that the existing contact would be updated to have the identifier provided that no other contact has that identifier and a match is otherwise made using the appropriate de-dupe rule