Thank you for adding this information. I have also just run across this issue, after finding that only six of the fifty trends returned contained tweet_volume data.
The API (https://dev.twitter.com/rest/reference/get/trends/place) very clearly states:
The tweet volume (tweet_volume) for the last 24 hours is also returned for every trend.
It’s difficult to understand why the implementation varies from Twitter’s own API documentation, and why the threshold is so arbitrary. The data must be available for all of them, otherwise they wouldn’t have been identified as trends in the first place… so it’s difficult to see how there could be a significant overhead for including the data. It’s a shame there was no further update from Andy.
I really hope Twitter looks at this again, as it prevents developers from doing any really interesting work of their own on trends that might add to what is available from Twitter’s own interface.