How to hold a keyframe still using an adjustment layer?

I have run a keyframe ramp to create a letter box. 

1. Drag adjustment layer in 
2. Drag in Letter box 
3. Set key frames 
4  Manually set my start and end point for the letterbox. 

However - the letterbox starts to reduce as the video plays but there is no other keyframe telling it do to that.?

See images 

Key:

Key frame 1 
Key frame 2 ( actual desired box / frame size 
Shot three several seconds later the box has reduced!?

I'm testing it and not getting that behavior, even with speed ramping on the main video clip. I do see in your screen shots that there's a keyframe for the offset parameter (image 3). But then, image 3 doesn't match the first two. I'd delete the Letterbox effect and redo it from scratch.