Deleting all products for fresh start, then importing a CSV with six cross-sell items per SKU produces dropped cross-sell rows.
No errors reported at import stage:
Checked rows: 4157, checked entities: 706, invalid rows: 0, total errors: 0 File is valid! To start import process press "Import" button
Re-exporting the data confirms that cross-sell rows are indeed lost - but total row count has increased to 4073, somehow.
Deleting all products then re-importing original CSV gives exact same results.
Store already successfully configured to cope with up to ten cross-sell items, as per How to Increase the Number of Crosssell Items in Cart?
Suggestions welcome :)