I experienced the exact same change in results as the OP describes: An effective 10x increase in CPA with the same ad copy and what I understood was the same targeting. When you think you're paying for the same thing but experience worse results by an order of magnitude, then you know something is up.
The deceptive nature of this change is that their new ad platform does not explain any of details that their product manager has graciously offered here. It's too bad that it has taken a front-page HN post to get this information to light; this already undermines the trust of the ads product team at Reddit. Thank you to the OP for explaining the change in laymen's terms.
Next, despite spending tens of thousands of dollars with Reddit, it's nearly impossible to be in contact with a competent person on their ads team. My support requests have gone routinely unanswered, or at best I receive a template response after days and days. After this most recent change I tried a last resort option of posting in the Reddit ads subreddit, which also received no response: https://www.reddit.com/r/redditads/comments/6wlyya/using_the...
The only benefit I've seen in the new platform is conversion tracking, which is a basic feature that has existed on even the most rudimentary ad systems for more than a decade. The Reddit Ads platform is way behind the times, which is sad because they're sitting on some of the most valuable web traffic in the world.