I have The Lounge hidden. Some threads from there are showing in the main feed.
(Edit)
Appears to be related to hitting the “reload latest” button.
Steps to recreate:
Start on the forum front page
Scroll down until the “reload latest” button appears.
Wait until a post has been made in a sub-forum you’ve hidden (Lounge, Sights and Sounds)
Press the “reload latest” button
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Your browser/os information:
Mozilla/5.0 (iPhone; CPU OS 13_6 like Mac OS X) AppleWebKit/605.1.15 (KHTML, like Gecko) FxiOS/27.0 Mobile/15E148 Safari/605.1.15 (this will help us make sense of your bug report)