So... I tweaked it a lot, hope it helps.
There are many cleaner cuts to the code and to the actors' hitboxes. I wasn't really sure where was the bug so I had to keep cleaning wherever I could. The bug now is gone, also the other bug is gone (the one that flickered the screen when your mouse got centered).
Also, maybe I found a Construct bug while fiddling with your code, so thanks for that.
It may turn out to not be a bug after all, but I'll update this thread when we get the results.
Either way, I'd recommend for you to check the file I posted in the bug report because it shows a very sleezy trap in the ELSE statement behavior.
Best of luck and feel free to ask anything.