I’ve just found a bug with filters and the “No items match the search” notice. It seems that the background of the window is not being completely cleared when a filtered text is displayed, so that the “No items match” notice is still visible when you change the filter from an empty one to another that have matches. I’ve made a video to expose the problem:
https://dl.dropboxusercontent.com/u/1279249/taskpaper-filters-nomatch-bug.mp4