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Did you check the settings tab of your block? The 'GroupName' in your code dictates which tab the property is displayed in, and it is set to 'Settings'.
Otherwise you could check the browser console for any errors.
I have a drop down property that looks like this:
When I create the block, the property shows up. But after the block has been created, it doesn't show up when I edit the block. This is in my local dev enviornment. It hasn't been published yet.