We are working on a fix for this in 5.11.3, you can try this out now if you like:
Edit the file ~/plugins/import_manager/components/migrators/whmcs/whmcs_migrator.php
Around line 70 look for the $actions code block, replace the entire block of code with the following (We moved importTransactions to below importInvoices.)
$actions = [
'importStaff', // works
'importClients', // works
'importContacts', // works
'importTaxes', // works
'importCurrencies', // works
'importPackages', // works
'importPackageOptions', // works
'importServices', // works
'importDomains', // works
'importInvoices', // works
'importTransactions', // works
'importSupportDepartments', // works
'importSupportTickets', // works
'importAffiliates', // works
'importMisc' // works
];
If you try this, let us know how it goes.