As far i have noticed, the question raised is somewhat the issue happened to me as well. I am surfing the Marketplace for quite days now and Magento as said have migrated the extensions from Magento connect as well.
I have extensions downloaded
instantsearch-autocompleteplus_autosuggest-3.0.12 which falls under the same category. FTP uploading is only option as mentioned.
But i feels the while migrating Magento might have misread the URL or required something from the extension providers which have lead to this issue. Magento 2 installs using composer which have the 'vendor/package' name combination to install. Whereas the Magento 1 extension do not have it, while migrating the Magento might have forced the extension / converted the extension to be written with 'Vendor/package name' which might have lead this issue.
Magento marketplace is still in early stages of integration, so it's a question whether the Marketplace migration has the fault since almost every extension migrated from Magento connect have the problem.