I recently made an effect material to distort my widget slightly, and I applied it through a retainer box. My hierarchy was like this:
Canvas Panel - Retainer Box with Effect - Canvas Panel - All my other widgets.
The problem is when I run my game events like Mouse Hover and Mouse Click are all out of line with my widgets. So if I have a button with a mouse hover event, I might hover my mouse underneath it and it would activate, or I might hover my mouse on it and nothing happens. Basically, the hitboxes for the buttons are not being affected by the distortion like everything else.
I am not sure why your problem is happening but when it happened to me i could fix it by enforcing that the game was running in a supported resolution (you can get a list of those during runtime from the object returned by the Get Game Settings node)