Ah. I just realized the issue with your specific setup (using Trigger Stay). The problem is that (by Unity), trigger stay (as well as enter ande exit), being a physics related action, it is not called in the normal update loop. This is also the reason why if you notice, in runtime debug mode, the connection out from Trigger Stay output, is not continuously active (showing the animation with the circles). As such, the fame in which you do press the “F” keyboard key (which is checked in the normal update loop), sometimes might be in sync with the physics loop, but more often than not, it will not. Having said that, I think that I will be able to make Trigger (and Collisions) Stay work in the normal update loop out of the box. If you can make a post in the forums about this, it would be great.
I have just improved this so that the Trigger and Collision Stay callbacks are in sync with the Update loop, and as such solve the problem you were having. If you want, I can send you the fix to your registered email. Let me know.