I don't know why it might return to the previous path on your machine but you might be able to brute force an update if you are comfortable editing the xml files contained inside your docm file.

I did a search in the xml files in a Word file and found two instances of the merge source path in the word\settings.xml file. The fact that there are two instances might be why making a change and saving isn't sticking for you.

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Andrew Lockton
Chrysalis Design, Melbourne Australia