The title pretty much explains everything: after a string gets activated, it will only unpower if the player is not looking at its hitbox!

What happened: the string got powered and remained in that state for as long as I looked at it.

What I expected to happen: I expected the string to unpower after a player steps away from it, regardless of whether he/she is looking at it or not.

How to replicate: 

1. Place a string
2. Open the F3 menu
3. Step on the string and look at it (check that it has powered)
4. Step away from it, while looking at the string the whole time (check that it hasn't unpowered)
5. Look away from the string
6. Look back at it (check that it has unpowered)

You can also check the video I made, where I am recording this bug (had  to upload to YouTube because I wasn't able to attach it to this bug report).