I am inclined to think it is something intervals needs to think about.
A couple of recent TrainerRoad workouts that upload to intervals are a second short there too. I have not noticed that happen previously.
I will post on the intervals forum about it.
Can you email me a fit file from TR that is good (correct seconds and one that is bad, 1 second off). Seeing this difference might spot why this is. Once I have those I will also research one more time with your workout you sent me from TD.
On closer inspection Alex, all the TR ones are showing 1 second short. The one I thought was correct that uploaded recently is not - I extended the cool down.
So I think it is something on intervals. I will add that to my post on their forum
A reply on the intervals forum states that activities uploaded from garmin and wahoo also show 1 second shorter duration on intervals and that this has always been the case