I’ve had the same issue as you where the input data doesn’t make a difference. I’ve also had the weird behavior where I would have to move selection with the keyboard before space bar works. Hitting tab makes it focus to the current button correctly (subsequent tabs move the focused element).
I currently have a work around using OnKeyDown and listening for a space bar input
This calls a c++ method in my derived UCommonButtonBase that calls HandleButtonClicked()
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Also, Common UI docs were updated. If anyone is looking to change the arrow key or space bar behaviors, the “Customize Navigation in Your UI” section here is useful