Giu
I had a similar thing a while back, but as a result of a modal form returning to its caller and the field on screen not updating (although debugging showed the value was changed). In my case it was how I was using the sc_redir which may not apply to you? See this link: http://www.scriptcase.net/forum/showthread.php?7796-Field-variable-won-t-update
That said, if that does not apply, I also had an issue (like a few others here) where an app was suddenly misbehaving and clearly corrupted somewhere. Re-genning had no effect so I rebuilt it from scratch (total pain!) but it worked! So that may be your (unfortunate) answer. 
What is interesting is that if you copy an app with a new name and re-gen it - the issue persists. This seems to indicate that it is one of the settings in the app that I changed causes the issue in these cases (or some weird corruption of how SC stores those setting values). But being there are soooooo many settings it is hard to know sometimes what setting may be at fault, and a fresh new app and re-gen is the only way to sometimes move forward - which does seem a bit like using a sledge hammer to crack a nut.
It would be very cool if SC could implement an audit log for every app somewhere of ALL changes (timestamped), e.g. app and field settings, event code, library changes, method code (php and javascrpt) made so they can be reviewed more easily. I appreciate this may get large pretty quickly, but as long as we have the power to choose the areas to be logged and clear it ourselves occasionally (or set a limit so it round robins perhaps) I’m sure that would prove invaluable!