Hi Vero,
As far as I understand TagEditor does not have a TrackChanges command.
So if you are editing something which is not in your TM it will appear as no match although the segment shows you a translated sentence.
There are no indications marks for changes in the target
... it means that the editor will have to look for the difference.
A nice workaround is to compare the 2 .ttx file with a file compare application and you will have the result
Cheers!