Advanced Search for activities ignores "OR" mode

On the Advanced Search page, switching to "OR" mode appears to have no effect on search results returned for the criteria in the Activities tab. The confirmation text returned does however confirm the intent to search in OR mode.

For example, with OR mode selected I searched for activities with Subject=Bob, Has Followup Activity=No, Activity Status=Cancelled. I got 0 results returned which is not the expected result as there were activities which met some of the criteria. The search actually performed seems to be an AND of the criteria, despite the confirmation message provided:

Activities without Followup Activities ...OR...
Subject Like %Bob% ...OR...
Activity Status In Cancelled