Is this mentioned in the documentation/Help???
I, too, have encountered that issue before, and your answer is the first explanation that I see.
John Di Rico wrote:
The progress bar does not increase if you commit an unedited fuzzy or unedited machine translation segment. It happens quite frequently that you don't have to edit a fuzzy match (e.g. a formatting difference). Currently, if you want it to progress, you can either add a space, remove it, then commit or use the "Verify Segment" icon (shortcut) or click on the green arrow in the far right column before committing.
This is confusing many users and I believe it will be changed in the next version.
So don't be confused by an incomplete progress bar.
I would contact tech support to see what the issue is. 9/10 times it is a tag but could be something special about the IDML file that corrupts the reconversion back.