This issue is only happening on my desktop Chrome, when a tweet is posted by anyone, it only appears as a link. For example:
I checked it out on Firefox and it's normal there:
Twitter embeds also work fine on my Pixel 2 phone's version of Chrome.
As an experiment I tried logging out of my account to see if it could be a forum user setting and the problem persists when not logged in. I don't have ad blocking turned on for any Penny-arcade sites.
I probably should have mentioned this bug when I first saw it but I assumed it was a temporary glitch and would come back, but now that it's been a few months I realize it probably won't come back without a bug fix.
Note that Youtube videos do not have the embed problem at all, it's only happening with tweets.