Separated Documents are sent with the same file name

I have "Keep documents separated" enabled on my enterprise account, but when the final signed PDFs are sent out, their original files names are stripped and replaced with the first document's name (such as "Cover Page") with a Part 1, Part 2, etc. instead. I find this to be a really poor experience for my customers, who now have to open all the files to locate the sepcific section they need. How can the original file names be retained?