Hey there,
I just ran into an issue EXACTLY the same as you described here. I had a settings widget that would not fire any events in the standalone game mode, but other menus worked (despite being setup in the same way).
Now this is where I don my tinfoil hat and tell you what fixed things for me, and I hope this helps you out as well.
Do you happen to copy and paste your blueprint code? For instance, if you were to add a new button to your UI, would you copy paste blueprint code that did the same thing from a previous button.. cuz for me that is what messed it up. I don’t know what kind of bug would cause this entanglement, but when I rolled back my changes, and then rebuilt the blueprint (the literal exact same way), without using copy-paste, then all events magically fired, and all the UI generated in standalone mode.
This is a longshot at best, but in the event this gets you, or others stumbling here in the future unstuck, I shall call it a win.
Good luck out there