In case anybody experiences this problem, this is how it was resolved.
Importing a course-file tar.gz gave this Upload error:
There was an error during the upload process. Some chunks missed during file upload. Please try again
The import was of a course export-file of >75MB (mostly due to un-optimised images).
The import-process breaks large imports into chunks of 20MB - I eventually found it mentioned in Maple Release Notes blog-post).
Attempting to import the >75MB course made Studio split the upload into 4 uploads (“chunks”) of <20MB; the failure of any 1 “chunk” (which could be at varying levels of completion between say 20% to 80%) would result in an import failure.
Workaround: By removing the 70+MB of over-sized assets from the course before exporting it,
the resulting 4MB course was imported without a problem and the optimised assets (same filenames) were bulk-uploaded separately into “Files & Uploads” for the newly-imported course.