I find this so very inconsistent it’s bordering on frustrating, the area of activation for the C4 seems tiny and the slightest deviation when trying to reposition while disarming causes you lose progress. I’ve also found the same with the EV as well. It got disabled in a match tonight, I was repairing away after being flashbanged only realise I was actually repairing it, I was just letting it see what my pliers looked like while activated. The disarm zone really needs increased or the disarm progress needs saved for about 3 seconds so you don’t lose it by looking a millimetre away from it. I get the same frustration I got with brink, I have to wait till it says I can arm/disarm/repair before it actually lets me.
Engineers Pliers
Mustang
(Mustang)
#2
The request has already been made that you should be able to look around 360 degrees whilst constructing, this should help a lot with these frustrations. Think of it as being able to assembly a weapon blindfolded, only in this case we are looking around rather than having the blindfold on. 
However I also agree that all construction progress should remain for a few seconds after disengaging and that is should decay over time rather than instantly.
I’ve also encountered a bug where during construction the progress gets stuck and won’t increase any further and the only way to get around this is to disengage and start again from scratch, which I find even more frustrating because it wasn’t even the players fault for looking away.