Hi Nevermore,
The issue you are seeing is caused by the Set Relative Location, running two timelines, and the values you are inputting for Z. Here is the crouching solution I use in my own projects, give it a try.
Create and set a variable to store the relative location of the camera at start. Do this in the Construction Script or the Event Graph using a EventBeginPlay node.
Then set this up in the Event Graph and reverse the Timeline on Release.
I hope that helps and if you have any questions feel free to post them here.
Thanks, TJ