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I had the same experience some time ago and I thought a bug was create with the plug-in team - the reference is JENSO-7765 and it's been open since Nov, 2015.
The work around is for the external user to save the file and upload it manually, as you said. If there are frequent simultaneous editors, you need to watch for merge issues.
Yikes! This really blows my whole collaboration strategy in our community. Am pushing Jive for Office hard.