This is no longer happening. It appears to have been fixed since it was reported.
How to repeat:
Android 11, Samsung Galaxy S10e
Chrome or Firefox
1. Go to a forum top level, like "Site Feedback".
2. Ensure page is not pinch zoomed in. No horizontal scrolling.
3. Tap a thread link.
4. Notice the page is zoomed in a tiny amount. The page thus scrolls horizontally a few pixels.
I can zoom out on the thread view page after it loads, but if I go up a level and then back in to a thread, it's zoomed in again ever so slightly.