As reported yesterday on the forum, there is a problem if you try to change trains between Kentish Town and Kentish Town West. Because the Underground station is closed, Kentish Town currently has no gates. Unfortunately the validators have the continuation exit setting so that if you touch one and then try to exit through the (currently closed) gates it doesn’t cause a problem. Continuation exit means that your journey will be kept open until you next exit through a gate or use a validator at a different station (or let the journey time out).
As an example, take a journey from Cricklewood to Canonbury. You touch in at Cricklewood, touch out at Kentish Town, walk to Kentish Town West and touch in, and finally touch out at Canonbury. Normally the Out-of-Station-Interchange (OSI) between the two Kentish Town stations will join the journeys into one. However, because Kentish Town West also has no gates, the system has no way of knowing whether your touch in there is really an in, or a further out. With continuation exit active it assumes it’s another exit and thus closes the journey from Cricklewood at Kentish Town West. You are then travelling to Canonbury on an unvalidated card and the exit there will be logged as an incomplete journey.
My advice for the moment is to avoid changing at Kentish Town from Thameslink. If you do need to do so you should touch out as usual, but when you get to Kentish Town West do not touch in. Your card is still validated for travel if continuation exit is set, and the correct fare will be charged when you then exit at your destination.
There is no problem in reverse because the validators at Kentish Town West will properly end your journey and the OSI will restart it when you touch in at Kentish Town.
I don’t offer this advice lightly, but if you are checked by a revenue inspector on the North London line you may be subject to a penalty fare or even threatened with prosecution. This is clearly not good as you haven’t done anything wrong.