Leni VERBOGEN 

What type of source files are you handling?  PDF files perhaps?  If you are it could be the result of not converting the PDFs to a DOCX prior to opening in Trados and cleaning them to remove the many superfluous entries that a PDF can contain.

No, I'm working with Word documents. In this case, the original has track changes.

Leni VERBOGEN 

I tested this with a DOCX containing tracked changes and don't have a problem.  So the issue is going to be related to the content of your DOCX.  Can you take a copy of the source file, save it somewhere else and then do this:

1. identify a paragraph that matches where you are seeing this problem.
2. delete everything in the document apart from this paragraph
3. open this reduced DOCX, save the target, and see if the same thing happens
4. if it does, change anything in that paragraph you consider confidential and make sure the problem still occurs with this changed version.
5. share that version by attaching to your reply
6. if it doesn't, look for another part of the document that does :-)

Then we can review the problem for real and it'll make it easier to see where the problem is coming from.

When I cut the text up into smaller pieces as you said, the problem doesn't occur...