SUMMARY
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Import an OFX statement (direct download or file) for an investment account, which includes a security not held in the account and you get a popup "The security XXXXX currently does not exist as a sub-account of Investment Account Name.  Do you want to create it?"
If you click YES, it creates that security account and completes the import fine.  If you click NO, it does not create that security account, but also does not import any further dividend transactions.  (probably not any other income transactions, but this OFX file has none.)  If does import the purchase transactions, further down in the OFX file.
(I don't want to create the new security because it actually already exists, but my bank has used a variant of the name for this transaction.)

To create an example, take a successfully imported OFX file with more than two dividend transactions and one or more buy transactions.  Change the SECNAME for the first dividend security in the OFX file to something not known, and import to a backup (so the transactions don't already exist)  No div tx past the first one will be imported, but the buy tx will be imported.