Activities cannot be searched using search box on App UI

We cannot search using the search box in Activities. Even if I am the owner, I cannot use it to search my own activities.

For example, when we are trying to search in search box using Title, there is no results found.

Even though we changed the operator to Equals, there is still no result.

However, when we are trying to search using the Saved Search 'My Open Activities', I can now search the activity.

What might be the issue on this?