AC4 example time issue

I am trying to debug a system i have built and have gone back to basics i am currently trying to understand how the machines work under DTC control so i am looking at the AC4 block.

however when using this block i notice an odd error. I input a square wave at 1 second for 1 second and the system executes as expected. If you input the same step at 2 seconds though (1 second later changing nothing else!) you get a completely different response.

has anyone come across this and have any idea whats going on?