wpForo is not based on custom post types, so you can't use AMP plugin for wpForo. We'll add built-in AMP functions in future releases.  Please disable the AMP plugin again, delete all caches and test it with a private session of mobile browser again.

If you still see the same result, disable other plugins and change the theme. This way you can find the problem maker. wpForo doesn't have any issue on mobile devices. Dozens of thousands websites use wpForo without any problem. This is AMP plugin issue, or other plugins and active theme issue, you can't see any change when you deactivate it, because of website and local browser cache.