Reddit is merely stepping into the big leagues.
What reddit has done is switched from "Contextual Category" (cx cat) targeting to "Behavioral" (bt) targeting. Usuaully behavioral is much much better, which is likely why the change was made. But OP was doing contextual cat really well, so it was more successful than bt.
Reddit made this switch because it will benefit the majority of advertisers, but hurt the ones who really know what they are doing. Reddit will earn more from the 80% that see improvement, and therefore spend more. The 20% like OP will spend less, but it won't matter.
CX CAT = Companies like grapeshot and peer39 use NLP to identify keywords on a webpage and categorize it. Then advertisers can buy ads on any webpage that is in those categories. One of my favorite strategies is to buy ads on content that is predicted to go viral in a certain category (like fashion). These are ads that HN usually would like because the ad content is similair to the content of the website. They target a webpage, not a user.
BT = Ads that target a specific user, no matter what website they're on. So once a user has visited r/jerky, they're tagged as liking beef jerky and could possibly see beef jerky ads on any site they visit. This is partially how you get ads that seems very out of place - beef jerky ads on a r/technology.
If you spend time buying media, you know reddit's behavior isn't surprising. Facebook and Google both have default settings that will burn through your budget and provide shitty results. Only someone who intimately knows the platform will be successful. Reddit is merely stepping into the big leagues.