You're incorrect, this is the normal way that the reddit algorithm functions, it promotes highly-engaging content from niche subreddits.
The top post of all time on r/toledo is about a police officer harassing a woman. It's highly engaging but not overtly political. It has 100x the upvotes of a normal r/toledo post about traffic or what have you.
One of the top posts of all time in r/sanjose is a video of someone trying to jimmy a hotel door open using a hook contraption. Highly engaging, not overtly political.
These were the first two city subreddits I checked. It's literally just how reddit works, highly engaging content bubbles to the top and can reach a much larger audience.
Why does it ONLY happen to political posts, and why does it ONLY happen to posts that reinforce a particular viewpoint?
Either this is the design of the black box "algorithm," or it's not real engagement. There's no need to miscorrect me about something so hamfisted and overt.
Yet political posts get +2000 out of nowhere and an influx of commentators who don't usually comment in that regional subreddit or don't even likely live there.