I found a handling. It accepts the following:

@Query(
   value = "SELECT MAX(CONVERT('TIMESTAMP', \"inv-date\") + (\"inv-time\" * 1000)) " +
   "rest of query...",
   nativeQuery = true
)
Optional<Date> findLastPaymentInvoiceDate(Long aAccountId);

So while I don't know why the odbc_convert command doesn't work, this convert command does work.