Not entirely sure, but I have some idea what’s going on:

Reddit redirects URLs of the form .reddit.com to www.reddit.com/r/subreddit, you can easily test that yourself by clicking this link: https://techsupport.reddit.com. However, if you take a “deeper” link like the link to this post and replace the www with a subreddit, you get something like this:

https://techsupport.reddit.com/r/techsupport/comments/182tb9i/what_the_hell_is_the110clubredditcom_that_i_get/

That link then redirects to www.reddit.com/r/techsupport/r/techsupport/, which reddit doesn’t like, and you just get an error page.

This seems to be broken for any subreddit with a number in it though. For example, https://the10thdentist.reddit.com just brings you to the old.reddit front page, like your example. And it also works with “deeper” links like posts: https://the10thdentist.reddit.com/r/techsupport/comments/182tb9i/what_the_hell_is_the110clubredditcom_that_i_get/. It also doesn’t have to be a real subreddit, anything with a number in it seems to result in the same behavior.

So… my guess is that someone at reddit fucked up these redirection URLs, and it results in Google wrongly indexing these links as if they were separate sites from old.reddit.com.