Tried using the search function in a thread and found that it doesn't produce any successful result - even with words found in the thread.

I'm using mobile Chrome to perform the search.

Same here. No results searching in Mikrotik thread: https://forum.lowyat.net/topic/2040480/

But is OK searching in Unifi Thread V26: https://forum.lowyat.net/topic/3974625

Tested a few other threads, some OK, some not.

There are some issues on the search index for the duration of the server upgrade and we are aware of the issue.