If a record to be imported contains a 999 with information that does not match the record to be overlaid, the data import fails silently in the UI, with an empty log. The log for mod-data-import shows the error:
If the UUIDs in the 999 ff $i and $s match the target record, the overlay succeeds. If there is no 999, the overlay succeeds. Interestingly, if there is a 999 with different indicator values (e.g. ##) and no 999 ff that matches, the overlay fails.
Tested with mod-data-import v1.10.2 and mod-source-record-manager v2.3.4.