Events not being picked up on a standalone build?

Hey there, I'm implementing iDropHandler within a script to detect an event on a 2D UI element. Running in the editor (on PC or Mac) it works perfectly - no errors, no issues.

However, as soon as I create a standalone build, the event isn't being intercepted. There's an event system in a persistant scene, I can't really think of what could cause it to work within the editor, but not in a standalone build (note that the issue is also occuring on an Android build). I'm using 2019.4.11f1.

Any thoughts? Thanks for any help!
Ant