It's a bug all right, the invasion map rotator is doing a poor job about setting the defense win time.
The patch has been submitted to Desura, it also concerns single player invasion. BMTMicro already has the update.
InvasionUS has got it fixed as of now.
If someone needs to get the fix before Desura gets the patch out for single player campaign, here it is:
http://modulaatio.com/runningwithrifles ... vasion.zip Unzip to RWR installation folder\media\packages\vanilla\scripts\trackers replacing the existing file.