An import only changes the columns you mapped

Two imports were writing where you hadn't asked. Updating brands from a file without a Websites column cleared the websites of every matched brand, and cleaning Fallback Stock on a product also cleared its On-hand Stock and Supplier Stock and stopped tracking the variant. Both are fixed: an update leaves unmapped columns alone, and cleaning Fallback Stock clears that value and nothing else. Values an earlier import cleared aren't put back, so check the brands you updated and the variants you cleaned. The steps are in Imports.