If you create two documents in the same folder, and add hyperlinks between them (just by browsing to the file each time, not by adding a full URL), then:
If you save the files as .mht (web page complete) then the hyperlinks will be absolute. If you rename the directory containing the files, the links will stop working.
If you save the files as .htm (web page) then the hyperlinks will be relative. This is surely what you want.
I doubt that this is a bug, but I don’t understand the rationale behind how it works. The sensible default, and the one used when saving as htm, is to make all of the local links relative. If you need to override this, you can type in the full path name instead of just selecting the file.