NOTE: This issue has been fixed. If you are experiencing cross-platform connection issues and there is no platform restricted content enabled on the world, see MCPE-36190
If platform restricted texture packs or resource packs are added to the world (not to Global Resources), the world will display on other platforms, identifying that it is join-able. However, attempting to join it results in a connection problem. Disabling the pack allows for cross-platform without errors.
I don't know how far back this issue goes, however I only just had it start happening after 1.16.100, as previously, the world would not show up on other platforms at all if platform restricted content was applied (which makes sense if cross-platform is unavailable) EDIT: Fixed in 1.16.220, worlds with platform restricted content no longer shows on other platforms.
Fixed in 1.16.220