I have logged into each UI. Take example of this community. In the default UI I could see one 3 hour old post which was specifically marked English but nothing else. After trying to fiddle with language settings thinking it could be that issue, later all posts disappeared. But Alexandrite showed everything.
I think I may have found out the cause. It is probably because default sorting is set to Top Day rather than Hot. If the community is low activity, there is no top post for that day and results are blank. Changing the sort type to Hot produces results. I think Alexandrite is the only UI with default set to Hot and rest are in Top Day mode probably.