"% Complete" Column Automatically Updating itself incorrectly

We have a persistant frustrating problem on only 1 sheet that is causing us significant issues. For some unknown reason, the WIP % (or "% Complete" as SS call it)

on a Gannt sheet seems to be mysteriously changing to 100%, but no-one is actually doing this.

It always seems to be a user called "VX Team Leader" (which is an active user within the company), and it always changes all children lines, and therefore the parent line too, to 100% (not any other % in between).

It often is doing it on jobs where we are changing the WIP % on other children lines, but i have overseen them updating it a number of times and they are not misinputting data. 

We have reset the password for "VX Team Leader", so if someone else was logged in as this user and was making the changes, that has now been ruled out. 

Does anyone know if this is a known issue, or if there is some typical known causes for this? Is there any formula or link from a report that may be writing back for some unknown reason? I have attached a screenshot of the changes happening on a typical job, and can share someone in to the sheet if that helps but does need sorting urgently, as it drives all our management figures & reports.